internal override ParsedSyntax CombineForPlus(ParsedSyntax other, BigRational otherValue) => other.CombineForPlus(this, otherValue);
internal override ParsedSyntax CombineForPlus(ParsedSyntax other) => other.CombineForPlus(this);
ParsedSyntax IAssociative.Combine(ParsedSyntax left, ParsedSyntax right) => left.CombineForPlus(right);