Consider the following Fibonacci function in Scheme:

$($define $($Fib n$)$

Write a function that calculates $n!$ based on the following recursive formula:

$n\{\begin{array}{cc}n(n-1)!,& n>0\\ 1,& n=0\end{array}$

Write a Scheme function that accurately implements the letter$-$grade cut$-$off points described in the syllabus using a function similar to "letter" defined this way:

Number $4$ as well please! Thank you!