[antlr-interest] Syntactic predicates vs branch-local declarations.

[r. clayton]

  I had never heard of it before

It comes from being anal retentive enough to always declare variables in the
smallest possible scope; see, for example, Section 10.4 of Code Complete (2nd
ed) by Steve McConnell.

Some confusion may come about because element labels are not branch local; the
rule

  binaryOp
    : Plus e1:exp e2:exp
    | Minus e1:exp e2:exp
    ;

causes problems:

  $ java antlr.Tool t.a
  ANTLR Parser Generator   Version 2.7.6 (20060528)   1989-2005
  t.a:5:11: Label 'e1' has already been defined
  t.a:5:18: Label 'e2' has already been defined
  Exiting due to errors.

  $

You have to resort to circumlocutions such as

  binaryOp
    : Plus pe1:exp pe2:exp
    | Minus me1:exp me2:exp
    ;

which is annoying.