When in doubt, type a question mark immediately after the call. Just type it. When in doubt, type a question mark at the tricky part. The second is the one we want. The second is the one we want.
Some calls can take a subcall in front, typically meaning to replace the first part.
The replacement call is placed in brackets. This is done in Sd by giving the command allow modifications before the call. Sd will ask about the various subcalls to be replaced. Answer y or n to the questions, and enter the selected subcalls. Whenever a call undergoes an interruption or enters or leaves a part with an additional concept on it, there is an implicit piecewise at that instant.