anchor.diff:  mult_expr -> mult_expr mult_op pow_expr :
   #binary_op{
     line=?line('$1'),
  -  type=op('$2'),
  +  type=?op('$2'),
     left='$1',
     right='$3'
   }.
  
positive.code: pow_expr -> pow_expr pow_op unary_expr :
  #binary_op{
    line=?line('$1'), 
    type=op('$2'), 
    left='$1', 
    right='$3'
  }.
  
negative.code: unary_expr -> unary_op unary_expr :
  #unary_op{
    line=?line('$1'),
    type=?op('$1'),
    val='$2'
  }.
  unary_expr -> call_expr : '$1'.
  