APOS does not error for missing definition of synthesized attribute conditionally assigned

[boyland]

See simple-binding2.aps: this example has a missing assignment of e.expr_shape in the variable case
(and the attribute has no default value):

  match ?e:Expr=variable(?id:String) begin
    decl : EntityRef;
    decl := lookup(id,e.expr_scope);
    if decl = not_found then
      msgs :> {id ++ " not declared"};
    else
      decl.entity_used :> true;
      e.expr_shape := decl.entity_shape;
    endif;
  end;

This omission should have been detected in analyze.