This is the opposite operation of extract_root_and_last_member():
the root and its supposed member are put together using the
$ separator, as in env_of_envs$env1$x, where the root and
the member could be either env_of_envs$env1 and x or
env_of_envs and env1$x.
collapse_root_and_member(root, member)String containing the root name to concatenate. It may be NULL or empty.
String containing the member name to concatenate. It may be NULL or empty.
A string concatenating the root and the member names with the
$ symbol. If any of them is empty or NULL, the other name is returned
or "" if the other name is also empty or NULL.
extract_root_and_last_member()