2.4 - multiple representations ... intro
2.4.1 - two representations for complex numbers
2.4.1 - add test cases
2.4.2 - re-impl using tags
2.4.2 - arithmetic ops with test
2.4.2 - some comments about the impl
2.4.3 - idea of data directed programming
2.4.3 - attempt to impl put
2.4.3 - impl get & put
move `get` and `put` to another file
2.4.3 - ben's impl
2.4.3 - alyssa's impl
2.4.3 - fix a misunderstanding to `put` and `get`
apply-generic
2.4.3 - pretty print for proc-table
2.4.3 - connect proc with impl
2.4.3 - tests from 2.4.2
ex 2.73 - reimpl deriv as deriv-1
ex 2.73.a - explanation
ex 2.73.b - done