add_topdown_callback: Add a function-specific callback to the top-down callbacks.
In mailund/foolbox: Function Manipulation Toolbox

Description Usage Arguments Details Value

This function adds to the existing topdown callback, rather than replace it, by putting a callback in front of it to be tested first. The callback will be invoked when the traversal sees a call to a specific function.

1	add_topdown_callback(callbacks, fn, cb)

`callbacks`	The existing callbacks.
`fn`	The function to which calls should be modified.
`cb`	The callback function to invoke.

The callback that is installed will be called with the usual callback arguments (which depend on context and user-provided information to ..., see rewrite_callbacks() and analysis_callbacks()), and additionally the next callback in line, through the parameter next_cb. This can be used to propagate information through several callbacks in a pipe-like fashion.