%!PS

% We want the 'n'th fibonacci number
/n 13 def

% Prepare output canvas:
/Helvetica findfont 20 scalefont setfont
100 100 moveto

%define the function recursively:
/fib {
  dup 2 lt
  { }
  {
    dup 1 sub fib
    exch 2 sub fib
    add
  } ifelse
} def

(Fib\() show n (....) cvs show (\) = ) show n fib (.....) cvs show

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