Problem 13 (150pts): cond

Fill in the missing parts of do_cond_form so that it correctly implements cond (spec), returning the value of the first result sub-expression corresponding to a true predicate, or the result sub-expression corresponding to else.

Some special cases:

• When the true predicate does not have a corresponding result sub-expression, return the predicate value.
• When a result sub-expression of a cond case has multiple expressions, evaluate them all and return the value of the last expression. (Hint: Use eval_all.)

Your implementation should match the following examples and the additional tests in tests.scm.

scm> (cond ((= 4 3) 'nope)
           ((= 4 4) 'hi)
           (else 'wait))
hi
scm> (cond ((= 4 3) 'wat)
           ((= 4 4))
           (else 'hm))
#t
scm> (cond ((= 4 4) 'here (+ 40 2))
           (else 'wat 0))
42

The value of a cond is undefined if there are no true predicates and no else. In such a case, do_cond_form should return None. If there is only an else, return its sub-expression. If it doesn't have one, return #t.

scm> (cond (False 1) (False 2))
scm> (cond (else))
#t

Use Ok to unlock and test your code:

python ok -q 13 -u
python ok -q 13