modcall: Auxiliary function: modifying calls
In ivmte: Instrumental Variables: Extrapolation by Marginal Treatment Effects

This function can be used to modify calls in several ways.

1	modcall(call, newcall, newargs, keepargs, dropargs)

`call`	Call object to be modified.
`newcall`	New function to be called.
`newargs`	List, new arguments and their values.
`keepargs`	List, arguments in original call to keep, with the rest being dropped.
`dropargs`	List, arguments in original call to drop, with the rest being kept.