add directory chapter-2 and sicp-2-1.ss
add segment constructor, selectors and midpoint-segment
add rectangle constructor, selectors, perimeter and area
an alternative procedural representation of pairs
add procedure representation of product 2^a * 3^b
add church numerals
add constructor and selectors for intervals
add subtract procedure for intervals
fixed the wrong bound
add procedure width and checking-width
add procedures add-interval, mul-interval, div-interval
add procedure div-interval
rewrite mul-interval with Bens suggestion
add constructor make-center-percent and selector percent
Demonstrate Lem's formula
load sicp-2-7.ss
add explianation of par1 and par2
to do it later
Merge branch 'feature/2-1' into develop
Merge branch 'develop' of github.com:DomeCharl-Wei/my-sicp into develop