overlay(s1,s2,p)             overlay s2 on s1

    Return string formed by overlaying string s1 with string s2 starting at
    position p. May extend length of s1.  If p > length(s1), s1 padded with
    blanks.

    rightm = "right margin";
    leftm  = "left margin";
    centers = "centered";

    s1 = center(centers,70,'-');
    -------------------------------centered--------------------------------

    s2 = overlay(s1,leftm,1);
    left margin--------------------centered--------------------------------

    s3 = overlay(s2,rightm,75 - length(rightm));
    left margin--------------------centered--------------------right margin