To evaluate a lambda expression in notes11.Ytrace.rkt:

0. Find a sub-expression of form:

( (lambda (x)   [expression1]) [expression2])

Reduce it (racket is convenient for this) by:

1. Placing markers to delimit sub-expression:

<( (lambda (x)   [expression1]) [expression2])>

2. Cut out [expression2]:

<( (lambda (x)   [expression1])    )>

3. Paste [expression2] for each occurence of x in [expression1]
(but nested lambda (x)s introduce a new level of scope).

4. Delete "<( (lambda (x)" and "))>".

5. Reformat (especially when [expression2] is significant).

6. Simplify arithmetic and list subexpressions along the way.

7. Generalizes for situations where lambda has multiple parameters in
place of x.