def expr_hash
      vary = valued_parameters.map do |key, value|
        [ key, to_expr(key, value) ]
      end

      Hash[ vary ]
    end
def expr_hash
      vary = valued_parameters.map do |key, value|
        [ key, to_expr(key, value) ]
      end

      Hash[ vary ]
    end
hash of keys to expressions, but only where
 there are values.