Undefined Varible is Parse Error

L.scala

object L {
  lazy val Spacing: P[Any] = ("\r" | "\t" | " " | "\r" | "\n").*
  def OPEN: P[Any] = "(" ~ Spacing
  def CLOSE: P[Any] = ")" ~ Spacing
  def EQ: P[Any] = "=" ~ Spacing
  def SEMI_COLON: P[Any] = string(";") ~ Spacing
  def VAL: P[Any] = "val" ~ Spacing
  lazy val S: P[Any] = Statements(!"") ~ !any
  def Statements(table: P[Any]): P[Any] = VAL ~ Identifier.evalCC{i => EQ ~ Expression(table) ~ SEMI_COLON ~ refer(Statements(table | i)).? } / Expression(table) ~ SEMI_COLON ~ refer(Statements(table)).?
  def Expression(table: P[Any]): P[Any] = refer(Primary(table)) ~ (range(Seq('+','-','*','/')) ~ Spacing ~ refer(Primary(table))).*
  def Primary(table: P[Any]): P[Any] =  table.and ~ Identifier | IntegerLiteral | (OPEN ~ refer(Expression(table)) ~ CLOSE)
  lazy val Identifier: P[Any] = range('a'to'z','A'to'Z',Seq('_')) ~ range('a'to'z','A'to'Z','0'to'9',Seq('_')).* ~ Spacing
  lazy val IntegerLiteral: P[Any] = range('0'to'9').+ ~ Spacing
}
val S = L.S
assert(S("(s * 1);").drop == ParseFailure("", "s * 1);"))
assert(S("val s = 1; (s * 1);").drop == ParseSuccess(None, ""))