Hello,

I am running into an antlr-mode bug when indenting long grammars.
Specifically, when the point in the grammar is higher than
antlr-slow-cache-diff-threshold indenting a grammar rule fails in
calling c-where-wrt-brace-construct through beginning-of-defun.

To trigger:
Write a longinsh grammar (more than 5k chars with the default setting)
or simply set antlr-slow-cache-diff-threshold to 0 and then try to
indent in a grammar rule.
See attached silly.g for an example.

Indentation breaks, with the following error:
c-where-wrt-brace-construct: c-beginning-of-decl-1 returned label

Backtrace:
  c-where-wrt-brace-construct()
  c-beginning-of-defun(1)
  funcall(c-beginning-of-defun 1)
  beginning-of-defun-raw(nil)
  ad-Orig-beginning-of-defun(nil)
  beginning-of-defun()
  antlr-syntactic-context()
  antlr-indent-line()
  antlr-indent-command(nil)
  call-interactively(antlr-indent-command nil nil)

Solution:
A simple solution (kludge) is to modify antlr-syntactic-context to not call
beginning-of-defun in antlr-mode.el.
See attached patch.