Hi Fabrizio, How are you? Its Friday, its me again :D. I realized some stuff but still don't really get the Expressions. Let me give you an example: exp := PPExpressionParser new. exp term: (($$ asParser trim , #letter asParser star) flatten / ($" asParser trim, $" asParser negate star, $" asParser trim) flatten); group: [ :g | g right: '=' asParser trim do: [:a :op :b | {a. op. b.}]. ]. exp parse: '$var = "dings"'. "this is valid code" exp parse: '"dings" = $var'. "this is invalid code" How do i tell the expression parser to make a constrain on one side? Or a better question could be: How do i combine expression parsers to achieve a similar behavior? Do i even have to? Or am I to assume that the programmer just knows the order...? I could do something like: group: [ :g | g right: '=' asParser trim do: [:a :op :b | { (LeftAssignmentSideExpressionParser new parse: a.) op. (RightAssignmentSideExpressionParser new parse: b.) } ]. ]. Do you see my Problem? Cheers Michael