Given an input function fn_name and a list of candidate functions funs_to_match, return a
list of all the functions in funs_to_match that appear in the definition of fn_name.
functions_called_by(fn_name, funs_to_match, where)A character vector listing the functions in funs_to_match that call fn_name.
<chr>
The name of the function of interest
<chr>
Only these functions will be considered as parents
<env>
An environment, or text specifying an environment