Check that script is being sourced
 

I have a script that must be sourced. Is there some way
to check that the script is being executed in this fashion
and throw an error if not?

assimilate() { source "$1" 2>/dev/null || echo "in ${FUNCNAME}(): \"$1\" not ${FUNCNAME}d "; }
]$ assimilate microsoft
in assimilate(): "microsoft" not assimilated