Helpers
    digit = ['0' .. '9'];

Tokens
    union = 'union';
    unitcircle = 'unitcircle';
    scale = 'scale';
    lpar = '(';
    rpar = ')';
    comma = ',' ;
    number = digit+;

Productions
  region = {unitcircle} unitcircle |
          {union} union lpar [r1]:region comma [r2]:region rpar |
          {scale} scale lpar number comma region rpar;