/fibstr {
    4 dict begin
    /limit exch def
    /b exch def
    /a exch def
    {
        /c a length b length add string def
        a c copy
        c a length b putinterval
        c =
        c length limit ge {
            exit
        } if
        /a b def
        /b c def
    } loop
    c limit 1 sub 1 getinterval
    end
} bind def

(1234) (5678) 51 fibstr (7) eq { (Pass) } { (FAIL) } ifelse =