Continuation and then try to implement McCarthy's amb-operator.

Continuation :

In computer science and programming, a continuation is an abstract representation of the control state of a computer program. A continuation reifies the program control state, i.e. the continuation is a data structure that represents the computational process at a given point in the process' execution; the created data structure can be accessed by the programming language, instead of being hidden in the runtime environment. It contains information such as the process' current stack (including all data whose lifetime is within the process e.g. "local variables"), as well as the process' point in the computation. An instance of continuation can be later used as a control structure; upon invocation, it will resume execution from the control point that it represents. The "current continuation" or "continuation of the computation step" is the continuation that, from the perspective of running code, would be derived from the current point in a program's execution.

and then try to implement McCarthy's Ambiguous Operator :

In 1963 John McCarthy, the inventor of Lisp, published the paper A Basis for a Mathematical Theory of Computation in which he proposed the function (in the computer program sense of the word) amb(.,.). The idea is that amb(x,y) is first equal to x. But if later in the computation it is found that this leads to some sort of contradiction the value of x is retracted and replaced with y. This is a much more complex business than it may seem to be at first. Retracting a value essentially means winding back the entire state of the computation to where it was when amb returned the value x, and then slipping in the value of y. This means somehow freezing and copying the entire state when x was first returned. When a contradiction is found the entire state of the program is discarded and replaced with the frozen version which is reactivated. These frozen states are known as continuations. In many ways it's like a GOTO statement on acid. It can cause a jump to an arbitrary spot on your code. But continuations are nicer than GOTOs because they are more amenable to logical reasoning.