code: func (g *Group) Insert(e *GroupExpr) bool {
  	if g.Exists(e) {
  		return false
  	}
  
  	operand := GetOperand(e.ExprNode)
  	var newEquiv *list.Element
  	mark, hasMark := g.FirstExpr[operand]
  	if hasMark {
  		newEquiv = g.Equivalents.InsertAfter(e, mark)
  	} else {
  		newEquiv = g.Equivalents.PushBack(e)
  		g.FirstExpr[operand] = newEquiv
  	}
  	g.Fingerprints[e.FingerPrint()] = newEquiv
  	return true
  }
docs: // Insert a nonexistent Group expression.