Fib := proc( n :: nonnegint )
        proc( k )
                option  remember; # automatically memoise
                if k = 0 then
                        0
                elif k = 1 then
                        1
                else
                        # Recurse, anonymously
                        thisproc( k - 1 ) + thisproc( k - 2 )
                end
        end( n )
end proc: