public override Task <NodeExpr> Visit(NodeExprUnary e, CancellationToken t) => Task.FromResult <NodeExpr>(e);
public virtual Task <T> Visit(NodeExprUnary e, CancellationToken t) => Task.FromResult(default(T));
public async override Task <string> Visit(NodeExprUnary e, CancellationToken t) => MathExpr.TypeUnaryStr[e.TypeUnary] + (await Visit(e.Expr, t));
public virtual T Visit(NodeExprUnary e) => default(T);
public NodeExprUnary(NodeExprUnary e) : this(e.TypeUnary, e.Expr)
{
}