Antlr stops parsing after the rule is completed, meaning that there may
be some syntax erros that could not be unit tested
e.g. task x := 5;o
the "o" would not be detected as a syntax error if the "task" rule is
completely parsed. the EOF token should help, however it does not: when
using the parser in interactive mode it does complain, but not in
standard parsing..
The workaround is to check the index of a possible following token after
parsing the rule. If it is not none, it means that the buffer still
contains something, which therefore is a syntax error.