def to_expr( key, value, field = nil )
      Sequel.expr( predicates[key, value, field] )
    end
def to_expr( key, value, field = nil )
      Sequel.expr( predicates[key, value, field] )
    end
turn a filter_parameter key => value into a Sequel::SQL::Expression subclass
 field will be the field name ultimately used in the expression. Defaults to key.