code: func builtinSetDiff(a, b ast.Value) (ast.Value, error) {
  
  	s1, err := builtins.SetOperand(a, 1)
  	if err != nil {
  		return nil, err
  	}
  
  	s2, err := builtins.SetOperand(b, 2)
  	if err != nil {
  		return nil, err
  	}
  
  	return s1.Diff(s2), nil
  }
docs: // Deprecated in v0.4.2 in favour of minus/infix "-" operation.