anchor.diff:  body : (body_literal COMMA | condition_literal SEMICOLON)* (body_literal | condition_literal) ;
   
   //体部文字
  -body_literal : literal | relation_expr | body_aggregate | NAF_NOT body_literal;
  +body_literal : literal | body_aggregate | NAF_NOT body_literal;
   
   //事实
   fact : head FULLSTOP;
  
positive.code: //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;
  
  //简单四则运算算术表达式，不加括号
  simple_arithmetic_expr :
  
negative.code:     CONSTANT LPAREN term (COMMA term)* RPAREN;
  
  //文字
  literal : atom | MINUS atom | NAF_NOT literal | comparison_literal;
  
  //缺省文字
  //default_literal : NAF_NOT literal;
  