In between each pair of results, stick in SEPARATOR. Thus, " " as
SEPARATOR results in spaces between the values returned by FUNCTION.
SEQUENCE may be a list, a vector, a bool-vector, or a string.
-SEPARATOR must be a string.
+SEPARATOR must be a string, a vector, or a list of characters.
FUNCTION must be a function of one argument, and must return a value
that is a sequence of characters: either a string, or a vector or
list of numbers that are valid character codepoints. */)