This routine inserts a character string (as many characters as will fit on the line) before the character under the cursor. All characters to the right of the cursor are shifted right with the possibility of the rightmost characters on the line being lost. The cursor position does not change (after moving to y, x, if specified). If n is specified, it inserts a leading substring of at most n characters, or if n<=0, then the entire string is inserted.
Special characters are handled as in
#f upon failure and
#t upon successful