add numbered? (stub) Chap06 p101 L1
modify numbered? (stub) Chap06 p102 L1
modify numbered? Chap06 p103 L1
modify numbered? Chap06 p103 L2
add value (stub) Chap06 p104 L4
modify value Chap06 p105 L1
modify value Chap06 p105 L7
add 1st-sub-exp Chap06 p107 L2
modify 1st-sub-exp Chap06 p107 L4
add 2nd-sub-exp Chap06 p107 L5
add operator Chap06 p107 L6
modify value Chap06 p108 L1
modify 1st-sub-exp, operator, value Chap06 p108 L3