How do I compose Linq Expressions? ie Func<Exp<Func<X, Y>>, Exp<Func<Y, Z>>, Exp<Func<X, Z>>>

I'm creating a Validator<T> class. I'm attempting to implement the Linq SelectMany extension methods for my validator to be able to compose expressions using a Linq query and validate the final result even when the underlying values change.

The following test code demonstrates my intent.

var a = 2;
var b = 3;

var va = Validator.Create(() => a, n => n >= 0 && n < 5);
var vb = Validator.Create(() => b, n => n >= 0 && n < 5);

var vc = from ia in va
         from ib in vb
         select ia + ib;

Debug.Assert(vc.Value == a + b); //2 + 3
Debug.Assert(vc.Value == 5);

Debug.Assert(vc.IsValid == true);

a = 7;

Debug.Assert(vc.Value == a + b); //7 + 3
Debug.Assert(vc.Value == 10);

Debug.Assert(va.IsValid == false);
Debug.Assert(vb.IsValid == true);
Debug.Assert(vc.IsValid == false);

I've seen the following question How do I compose existing Linq Expressions which shows me how to compose two Func<T, bool>'s together using an And expression, but I need to be able to compose functions together in a more, well, functional way.

I have, for example, the following two expressions:

public Expression<Func<T>> ValueExpression { get; private set; }
public Expression<Func<T, bool>> ValidationExpression { get; private set; }

I wish to create a new expression like this:

    public Expression<Func<bool>> IsValidExpression
    {
        get
        {
            // TODO: Compose expressions rather than compile & invoke.
        }
    }

More succinctly I'm trying to create these functions:

// Specific case
Func<Expression<Func<T>>, Expression<Func<T, bool>>, Expression<Func<bool&g