def expr_for( predicate, field = nil )
      unless (value = valued_parameters[predicate]).blank?
        to_expr( predicate, value, field )
      end
    end
def expr_for( predicate, field = nil )
      unless (value = valued_parameters[predicate]).blank?
        to_expr( predicate, value, field )
      end
    end
turn the expression at predicate into a Sequel expression with
 field, having the value for predicate. Will be nil if the
 predicate has no value in valued_parameters.
 Will always be a Sequel::SQL::Expression.