diff --git a/lib/Language/Bel/Compiler.pm b/lib/Language/Bel/Compiler.pm
index d613dc01..05c0f66f 100644
--- a/lib/Language/Bel/Compiler.pm
+++ b/lib/Language/Bel/Compiler.pm
@@ -4,7 +4,7 @@ use 5.006;
 use strict;
 use warnings;
 
-use Language::Bel::Compiler::Pass::AllocateRegisters;
+use Language::Bel::Compiler::Pass::Allocate;
 use Language::Bel::Compiler::Pass::Alpha;
 use Language::Bel::Compiler::Pass::Flatten;
 use Language::Bel::Compiler::Generator qw(
@@ -22,7 +22,7 @@ sub make_pass {
 my @PASSES = map { make_pass($_) } qw<
     Alpha
     Flatten
-    AllocateRegisters
+    Allocate
 >;
 
 sub compile {
diff --git a/lib/Language/Bel/Compiler/Gensym.pm b/lib/Language/Bel/Compiler/Gensym.pm
index ae1c43cd..003d8d40 100644
--- a/lib/Language/Bel/Compiler/Gensym.pm
+++ b/lib/Language/Bel/Compiler/Gensym.pm
@@ -19,12 +19,17 @@ sub gensym {
     return $GENSYM_PREFIX . sprintf("%04d", ++$unique_gensym_index);
 }
 
+sub starts_with {
+    my ($string, $prefix) = @_;
+
+    return substr($string, 0, length($prefix)) eq $prefix;
+}
+
 sub is_gensym {
     my ($expr) = @_;
 
     return is_symbol($expr)
-        && substr(symbol_name($expr), 0, length($GENSYM_PREFIX))
-            eq $GENSYM_PREFIX;
+        && starts_with(symbol_name($expr), $GENSYM_PREFIX);
 }
 
 our @EXPORT_OK = qw(
diff --git a/lib/Language/Bel/Compiler/Pass.pm b/lib/Language/Bel/Compiler/Pass.pm
index 6653e089..f51d4473 100644
--- a/lib/Language/Bel/Compiler/Pass.pm
+++ b/lib/Language/Bel/Compiler/Pass.pm
@@ -12,13 +12,31 @@ sub new {
     }, $class);
 }
 
-# abstract
 sub translate {
+    my ($self, $program) = @_;
+
+    $self->check_precondition($program);
+    my $result = $self->do_translate($program);
+    $self->check_postcondition($result);
+
+    return $result;
+}
+
+sub check_precondition {
+    # do nothing by default
+}
+
+sub check_postcondition {
+    # do nothing by default
+}
+
+# @abstract
+sub do_translate {
     my ($self) = @_;
 
     my $name = $self->{name};
 
-    die "The [$name] pass doesn't implement a 'translate' method";
+    die "The [$name] pass doesn't implement a 'do_translate' method";
 }
 
 1;
diff --git a/lib/Language/Bel/Compiler/Pass/Allocate.pm b/lib/Language/Bel/Compiler/Pass/Allocate.pm
new file mode 100644
index 00000000..0e3d403c
--- /dev/null
+++ b/lib/Language/Bel/Compiler/Pass/Allocate.pm
@@ -0,0 +1,187 @@
+package Language::Bel::Compiler::Pass::Allocate;
+use base qw(Language::Bel::Compiler::Pass);
+
+use 5.006;
+use strict;
+use warnings;
+
+use Language::Bel::Core qw(
+    is_nil
+    is_pair
+    is_symbol
+    is_symbol_of_name
+    make_pair
+    make_symbol
+    symbol_name
+);
+use Language::Bel::Printer qw(
+    _print
+);
+use Language::Bel::Compiler::Gensym qw(
+    is_gensym
+);
+use Language::Bel::Compiler::Primitives qw(
+    car
+    cdr
+);
+
+sub new {
+    my ($class) = @_;
+
+    return $class->SUPER::new("allocate-registers");
+}
+
+sub substitute_registers {
+    my ($expr) = @_;
+
+    if (is_symbol($expr) && is_gensym($expr)) {
+        return make_symbol("%0");
+    }
+    elsif (is_pair($expr)) {
+        return make_pair(
+            substitute_registers(car($expr)),
+            substitute_registers(cdr($expr)),
+        );
+    }
+    else {
+        return $expr;
+    }
+}
+
+sub starts_with {
+    my ($string, $prefix) = @_;
+
+    return substr($string, 0, length($prefix)) eq $prefix;
+}
+
+my $REGISTER_PREFIX = "%";
+
+sub is_register {
+    my ($expr) = @_;
+
+    return is_symbol($expr)
+        && starts_with(symbol_name($expr), $REGISTER_PREFIX);
+}
+
+# @override
+sub check_precondition {
+    # no checks
+}
+
+# @override
+sub do_translate {
+    my ($self, $ast) = @_;
+
+    $ast = cdr($ast);
+    my $args = substitute_registers(car($ast));
+
+    my $body = cdr($ast);
+
+    return make_pair(
+        make_symbol("def-03"),
+        make_pair(
+            $args,
+            substitute_registers($body),
+        ),
+    );
+}
+
+# @override
+sub check_postcondition {
+    my ($self, $ast) = @_;
+
+    my $body = cdr(cdr($ast));
+
+    while (!is_nil($body)) {
+        die "body not a pair"
+            unless is_pair($body);
+
+        my $operation = car($body);
+
+        my @operands;
+        while (!is_nil($operation)) {
+            die "operation is not a pair"
+                unless is_pair($operation);
+            push @operands, car($operation);
+            $operation = cdr($operation);
+        }
+
+        my $op_name = shift(@operands);
+
+        if (is_symbol_of_name($op_name, "return")) {
+            die "expected gensym as only operand to 'return'"
+                unless scalar(@operands) == 1 && is_register($operands[0]);
+        }
+        elsif (is_register($op_name)) {
+            die "expected two operands"
+                unless scalar(@operands) == 2;
+            die "expected := immediately after gensym"
+                unless is_pair($operands[0])
+                    && is_symbol_of_name(car($operands[0]), "compose")
+                    && is_symbol_of_name(car(cdr($operands[0])), "=");
+
+            my $rhs = $operands[1];
+            my @rhs_operands;
+            while (!is_nil($rhs)) {
+                die "rhs operation is not a pair"
+                    unless is_pair($rhs);
+                push @rhs_operands, car($rhs);
+                $rhs = cdr($rhs);
+            }
+
+            my $rhs_op = shift(@rhs_operands);
+
+            if (is_pair($rhs_op)
+                && is_symbol_of_name(car($rhs_op), "prim")
+                && is_pair(car(cdr($rhs_op)))
+                && is_symbol_of_name(car(car(cdr($rhs_op))), "quote")
+                && is_symbol(car(cdr(car(cdr($rhs_op)))))) {
+
+                my $primop_name = symbol_name(car(cdr(car(cdr($rhs_op)))));
+
+                die "expected primop to be id or type"
+                    unless $primop_name eq "id" || $primop_name eq "type";
+
+                if ($primop_name eq "id") {
+                    die "expected prim!id to have 2 operands"
+                        unless scalar(@rhs_operands) == 2;
+
+                    die "expected first prim!id operand to be a register"
+                        unless is_register($rhs_operands[0]);
+
+                    die "expected second prim!id operand to be a quoted symbol"
+                        unless is_pair($rhs_operands[1])
+                            && is_symbol_of_name(car($rhs_operands[1]), "quote")
+                            && is_symbol(car(cdr($rhs_operands[1])));
+                }
+                elsif ($primop_name eq "type") {
+                    die "expected prim!type to have 1 operand"
+                        unless scalar(@rhs_operands) == 1;
+
+                    die "expected prim!type operand to be a register"
+                        unless is_register($rhs_operands[0]);
+                }
+                else {
+                    die "unexpected type of primop: $primop_name";
+                }
+            }
+            elsif (is_symbol_of_name($rhs_op, "quote")) {
+                die "expected quote to have 1 operand"
+                    unless scalar(@rhs_operands) == 1;
+
+                die "expected quote operand to be a symbol"
+                    unless is_symbol($rhs_operands[0]);
+            }
+            else {
+                die "unexpected rhs";
+            }
+        }
+        else {
+            die "unrecognized operation: ", _print(car($body));
+        }
+
+        $body = cdr($body);
+    }
+}
+
+1;
diff --git a/lib/Language/Bel/Compiler/Pass/AllocateRegisters.pm b/lib/Language/Bel/Compiler/Pass/AllocateRegisters.pm
deleted file mode 100644
index d2dbba94..00000000
--- a/lib/Language/Bel/Compiler/Pass/AllocateRegisters.pm
+++ /dev/null
@@ -1,62 +0,0 @@
-package Language::Bel::Compiler::Pass::AllocateRegisters;
-use base qw(Language::Bel::Compiler::Pass);
-
-use 5.006;
-use strict;
-use warnings;
-
-use Language::Bel::Core qw(
-    is_pair
-    is_symbol
-    make_pair
-    make_symbol
-);
-use Language::Bel::Compiler::Gensym qw(
-    is_gensym
-);
-use Language::Bel::Compiler::Primitives qw(
-    car
-    cdr
-);
-
-sub new {
-    my ($class) = @_;
-
-    return $class->SUPER::new("allocate-registers");
-}
-
-sub substitute_registers {
-    my ($expr) = @_;
-
-    if (is_symbol($expr) && is_gensym($expr)) {
-        return make_symbol("%0");
-    }
-    elsif (is_pair($expr)) {
-        return make_pair(
-            substitute_registers(car($expr)),
-            substitute_registers(cdr($expr)),
-        );
-    }
-    else {
-        return $expr;
-    }
-}
-
-sub translate {
-    my ($self, $ast) = @_;
-
-    $ast = cdr($ast);
-    my $args = substitute_registers(car($ast));
-
-    my $body = cdr($ast);
-
-    return make_pair(
-        make_symbol("def-03"),
-        make_pair(
-            $args,
-            substitute_registers($body),
-        ),
-    );
-}
-
-1;
diff --git a/lib/Language/Bel/Compiler/Pass/Alpha.pm b/lib/Language/Bel/Compiler/Pass/Alpha.pm
index 2af70df9..1e94da2e 100644
--- a/lib/Language/Bel/Compiler/Pass/Alpha.pm
+++ b/lib/Language/Bel/Compiler/Pass/Alpha.pm
@@ -9,6 +9,7 @@ use Language::Bel::Core qw(
     is_nil
     is_pair
     is_symbol
+    is_symbol_of_name
     make_pair
     make_symbol
     symbol_name
@@ -23,6 +24,7 @@ use Language::Bel::Compiler::Primitives qw(
 );
 use Language::Bel::Compiler::Gensym qw(
     gensym
+    is_gensym
 );
 
 sub new {
@@ -48,19 +50,27 @@ sub replace_variables {
     }
 }
 
-sub translate {
+# @override
+sub check_precondition {
     my ($self, $ast) = @_;
 
-    $ast = cdr($ast);
-    my $fn_name = car($ast);
-
-    $ast = cdr($ast);
-    my $args = car($ast);
+    my $args = car(cdr(cdr($ast)));
 
     die "Not general enough to handle these args yet: ", _print($args)
         unless is_pair($args)
             && is_symbol(car($args))
             && is_nil(cdr($args));
+}
+
+# @override
+sub do_translate {
+    my ($self, $ast) = @_;
+
+    $ast = cdr($ast);
+    my $fn_name = car($ast);
+
+    $ast = cdr($ast);
+    my $args = car($ast);
 
     my $single_param_name = symbol_name(car($args));
 
@@ -86,4 +96,51 @@ sub translate {
     );
 }
 
+# @override
+sub check_postcondition {
+    my ($self, $ast) = @_;
+
+    my $args = car(cdr(cdr($ast)));
+    my $body = cdr(cdr(cdr($ast)));
+
+    assure_meets_postcondition($body);
+}
+
+sub is_among_symbol_names {
+    my ($value, @names) = @_;
+
+    for my $name (@names) {
+        return 1
+            if is_symbol_of_name($value, $name);
+    }
+    return '';
+}
+
+sub assure_meets_postcondition {
+    my ($value) = @_;
+
+    if (is_nil($value)) {
+        return;
+    }
+    elsif (is_pair($value) && is_symbol_of_name(car($value), "quote")) {
+        return;
+    }
+    elsif (is_pair($value)) {
+        assure_meets_postcondition(car($value));
+        assure_meets_postcondition(cdr($value));
+    }
+    elsif (is_gensym($value)) {
+        return;
+    }
+    elsif (is_among_symbol_names($value, "id", "type")) {
+        return;
+    }
+    elsif (is_symbol_of_name($value, "no")) {
+        return;         # XXX: should be generalized to "known globals"
+    }
+    else {
+        die "Doesn't meet precondition: '", _print($value), "'";
+    }
+}
+
 1;
diff --git a/lib/Language/Bel/Compiler/Pass/Flatten.pm b/lib/Language/Bel/Compiler/Pass/Flatten.pm
index 97b700ce..ba6bfb47 100644
--- a/lib/Language/Bel/Compiler/Pass/Flatten.pm
+++ b/lib/Language/Bel/Compiler/Pass/Flatten.pm
@@ -23,6 +23,7 @@ use Language::Bel::Reader qw(
 );
 use Language::Bel::Compiler::Gensym qw(
     gensym
+    is_gensym
 );
 use Language::Bel::Compiler::Primitives qw(
     car
@@ -135,7 +136,22 @@ sub listify {
     return $list;
 }
 
-sub translate {
+# @override
+sub check_precondition {
+    my ($self, $ast) = @_;
+
+    my $body = cdr(cdr(cdr($ast)));
+
+    while (!is_nil($body)) {
+        my $statement = car($body);
+        die "Can't handle 'if' statements just yet"
+            if is_pair($statement) && is_symbol_of_name(car($statement), "if");
+        $body = cdr($body);
+    }
+}
+
+# @override
+sub do_translate {
     my ($self, $ast) = @_;
 
     $ast = cdr($ast);
@@ -171,4 +187,102 @@ sub translate {
     );
 }
 
+# @override
+sub check_postcondition {
+    my ($self, $ast) = @_;
+
+    my $body = cdr(cdr($ast));
+
+    while (!is_nil($body)) {
+        die "body not a pair"
+            unless is_pair($body);
+
+        my $operation = car($body);
+
+        my @operands;
+        while (!is_nil($operation)) {
+            die "operation is not a pair"
+                unless is_pair($operation);
+            push @operands, car($operation);
+            $operation = cdr($operation);
+        }
+
+        my $op_name = shift(@operands);
+
+        if (is_symbol_of_name($op_name, "return")) {
+            die "expected gensym as only operand to 'return'"
+                unless scalar(@operands) == 1 && is_gensym($operands[0]);
+        }
+        elsif (is_gensym($op_name)) {
+            die "expected two operands"
+                unless scalar(@operands) == 2;
+            die "expected := immediately after gensym"
+                unless is_pair($operands[0])
+                    && is_symbol_of_name(car($operands[0]), "compose")
+                    && is_symbol_of_name(car(cdr($operands[0])), "=");
+
+            my $rhs = $operands[1];
+            my @rhs_operands;
+            while (!is_nil($rhs)) {
+                die "rhs operation is not a pair"
+                    unless is_pair($rhs);
+                push @rhs_operands, car($rhs);
+                $rhs = cdr($rhs);
+            }
+
+            my $rhs_op = shift(@rhs_operands);
+
+            if (is_pair($rhs_op)
+                && is_symbol_of_name(car($rhs_op), "prim")
+                && is_pair(car(cdr($rhs_op)))
+                && is_symbol_of_name(car(car(cdr($rhs_op))), "quote")
+                && is_symbol(car(cdr(car(cdr($rhs_op)))))) {
+
+                my $primop_name = symbol_name(car(cdr(car(cdr($rhs_op)))));
+
+                die "expected primop to be id or type"
+                    unless $primop_name eq "id" || $primop_name eq "type";
+
+                if ($primop_name eq "id") {
+                    die "expected prim!id to have 2 operands"
+                        unless scalar(@rhs_operands) == 2;
+
+                    die "expected first prim!id operand to be a gensym"
+                        unless is_gensym($rhs_operands[0]);
+
+                    die "expected second prim!id operand to be a quoted symbol"
+                        unless is_pair($rhs_operands[1])
+                            && is_symbol_of_name(car($rhs_operands[1]), "quote")
+                            && is_symbol(car(cdr($rhs_operands[1])));
+                }
+                elsif ($primop_name eq "type") {
+                    die "expected prim!type to have 1 operand"
+                        unless scalar(@rhs_operands) == 1;
+
+                    die "expected prim!type operand to be a gensym"
+                        unless is_gensym($rhs_operands[0]);
+                }
+                else {
+                    die "unexpected type of primop: $primop_name";
+                }
+            }
+            elsif (is_symbol_of_name($rhs_op, "quote")) {
+                die "expected quote to have 1 operand"
+                    unless scalar(@rhs_operands) == 1;
+
+                die "expected quote operand to be a symbol"
+                    unless is_symbol($rhs_operands[0]);
+            }
+            else {
+                die "unexpected rhs";
+            }
+        }
+        else {
+            die "unrecognized operation: ", _print(car($body));
+        }
+
+        $body = cdr($body);
+    }
+}
+
 1;