Compiles input text file, input.txt, following the PL/0 grammar:
program = block "." .
block = [ "const" ident "=" number {"," ident "=" number} ";"] [ "var" ident {"," ident} ";"] { "procedure" ident ";" block ";" } statement .
statement = [ ident ":=" expression | "call" ident | "?" ident | "!" expression | "begin" statement {";" statement } "end" | "if" condition "then" statement | "while" condition "do" statement ].
condition = "odd" expression | expression ("="|"#"|"<"|"<="|">"|">=") expression .
expression = [ "+"|"-"] term { ("+"|"-") term}.
term = factor {("*"|"/") factor}.
factor = ident | number | "(" expression ")".