When parsing parse.y with the `--report=cex` option, the process does not finish

[ydah]

How to reproduce

1. check out this branch: https://github.com/ydah/ruby/tree/use-user-defined-parameterizing-rule

2. run following commands

$ ruby tool/id2token.rb parse.y > parse.tmp.y   
$ tool/lrama/exe/lrama --report=cex parse.tmp.y

Investigation

Works fine without the option.

$ exe/lrama parse.t.y
shift/reduce conflicts: 3 found, 0 expected

And if it is parse.y in the master branch, it works fine.