public static SymbolicExpression Add(ImmutableArray <SymbolicExpression> operands)
 => ExpressionSimplifier.TrySimplifyAddition(operands)
 ?? new VariadicOperationExpression(VariadicOperator.Addition, operands);
 public static SymbolicExpression Add(SymbolicExpression a, SymbolicExpression b)
 => ExpressionSimplifier.TrySimplifyAddition(a, b)
 ?? new BinaryOperationExpression(BinaryOperator.Addition, a, b);