Add precedence operator to sequence definition

[jvdb]

For example:

sequence
  (a > b)*

Would then mean iteration over either a and/or b, but attempt to match a first. This is especially useful when dealing with specialized instances of existing structures. Currently this is sometimes done:

sequence
  (a b c)*
  (a b d)*

When what was really intended was:

sequence
  (a b c > d)*

Note: These two are not equivalent, but that's part of the problem.

[jvdb]

An alternative solution is to make the order in which tokens appear significant. So the last example would then be:

sequence
  (a b c d)*

Which would then explicitly mean that d is to be attempted last.