flow_draw() should only be used in the debugger triggered by a call
to flow_run(), or following a call to flow_debug().
d is an active binding to flow_draw(), it means you can just type d
(without parentheses) instead of flow_draw().
flow_draw()
dReturns NULL invisibly (called for side effects)
d was designed to look like the other shortcuts detailed in ?browser,
such as f, c etc... It differs however in that it can be overridden.
For instance if the function uses a variable d or that a parent environment
contains a variable d, flow::d won't be found. In that case you will
have to use flow_draw().
If d or flow_draw() are called outside of the debugger they will return
NULL silently.