subst
(subst new_element old_element list)
The subst function copies a list substituting every occurrence of new_element for old_element.
If old_element cannot be found in the given list, subst returns the original list unchanged.
: (subst 1 4 '(4 2 3 4 2 3 4 2 3))
(1 2 3 1 2 3 1 2 3)
: (subst 1 4 '(1 2 3 1 2 3 1 2 3))
(1 2 3 1 2 3 1 2 3)
getstring