We have the following grammar for a simple language:

  stmts^: stmt+;

  stmt: nop | if | while | assignment;
  nop^: '';
  if: 'if'^ expr 'then'! stmts ('else'! stmts)? 'end'!;
  while: 'while'^ expr 'do'! stmts 'end'!;
  assignment: IDENT '='^ expr;

  expr: term (('+'^|'-'^) term)*;
  term: atom (('*'^|'/'^) atom)*;
  atom: '-'^ atom | IDENT | NATURAL_LIT | '('! expr ')'!;

  IDENT: ('a'..'z'|'A'..'Z'|'_')('a'..'z'|'A'..'Z'|'_'|'0'..'9')*;
  NATURAL_LIT: ('0'..'9')+;

However, we have detected that one of the loops might hang. This happens when a certain lookahead is valid to enter a loop, but this loop does not consume anything with this lookahead. Change the grammar to fix this problem, without modifying the language or AST construction.