add: 2_05.scm
add: 2_06.scm
add: interval.scm 2_0{7,8}.scm
add: 2_10.scm
updt: 2_08.scm
add: 2_09.scm
add: 2_11.scm
add: make-center-width.scm 2_12.scm
add: 2_13.scm
add: parallel-resistor.scm 2_14.scm