anchor.diff:  //数字
   bit_number : unary_op? natural_number;
   //关系运算符(comparison predicates)
  -relation_op : LESS_THAN | LEQ | GREATER_THAN | GEQ | EQUAL | DOUBLE_EQUAL | NEQ;
  +relation_op : LESS_THAN | LEQ | GREATER_THAN | GEQ | EQUAL | NEQ;
   
   //简单四则运算算术表达式，不加括号
   simple_arithmetic_expr :
  
positive.code:     CONSTANT LPAREN term (COMMA term)* RPAREN;
  
  //文字
  literal : atom | MINUS atom | NAF_NOT literal;
  
  //缺省文字
  //default_literal : NAF_NOT literal;
  
negative.code: body : (body_literal COMMA | condition_literal SEMICOLON)* (body_literal | condition_literal) ;
  
  //体部文字
  body_literal : literal | body_aggregate | NAF_NOT body_literal;
  
  //事实
  fact : head FULLSTOP;
  