Type signature OutputSig::VariableWriteSourceNode
A node representing the incoming value about to be written at the given assignment.
The captured-variable library will generate flows out of this node, and assume that other parts of the language implementation produce the relevant data flows into this node.
For ordinary assignments, this could be mapped to the right-hand side of the assignment.
For more general cases, where an lvalue has no direct corresponding rvalue, this can be mapped to a data-flow node that wraps the lvalue, with language-specific incoming data flows.