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Part. Tail Recursion and Iteration. See also
the paragraph Fibonacci function in the section Primitive Recursion and
the paragraph Simulation of for-loops by recursion in the section Course of Values Recursion.
Your solution should simulate the following while-loop by primitive recursion:
if n = 0 then return(0) else begin n,a,b := n-1,1,0; while n > 0 do n,a,b := n-1,a+b,a; return(a) end