Get the first or all references of an R object
Assert that there are no references among the identified globals
The R object to be checked.
TRUE, only the first reference is returned,
otherwise all references.
Type of action to take if a reference is found.
Is the source of
x the globals or the value of the future?
find_references() returns a list of zero or more references
If a reference is detected, an informative error, warning, message,
or a character string is produced, otherwise
NULL is returned