@Override
			public ErrorManager getErrorManager() { return errMgr; }
Important enough to avoid multiple definitions that we do very early,
right after AST construction. Also check for undefined rules in
parser/lexer to avoid exceptions later. Return true if we find multiple
definitions of the same rule or a reference to an undefined rule or
parser rule ref in lexer rule.
tool/src/org/antlr/v4/Tool.java