code: func (expr *Expr) Operand(pos int) *Term {
  	terms, ok := expr.Terms.([]*Term)
  	if !ok {
  		return nil
  	}
  	idx := pos + 1
  	if idx < len(terms) {
  		return terms[idx]
  	}
  	return nil
  }
docs: // Operand returns the term at the zero-based pos. If the expr does not include
  // at least pos+1 terms, this function returns nil.