mapconcat

mapconcat is a built-in function in `C source code'.

(mapconcat FUNCTION SEQUENCE SEPARATOR)

Apply FUNCTION to each element of SEQUENCE, and concat the results as strings.
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.