Model evaluation of graphs with multiple returns

[robrix]

Specifically, the semantics must avoid redundant computation of shared dependencies. For example, if we had two inputs, a and b, and returned two values, a + b - a and a + b - b, a + b must be computed exactly once.

[robrix]

Currently I’m thinking of a graph with n returns as n functions (at least externally).

[robrix]

This would be greatly simplified by #29. By-need evaluation means that a graph with multiple returns has a product return type, and we only compute the fields which we project out of it.