domain = R -> D \equiv R

A function is real-valued when the domain is equivalent to the set of
real numbers. I am not sure equals is preferable here.
Makes proof a bit more clear

Brings truth table within the example and makes sure the two are placed
near each other.
Clarifies example about operator precedence
Boldface example/defn names

Used to be small caps. Was ugly.
Clarifies set construction wording/notation
Renames chapter to "Functions"
Formal definition of functions
`=` is the equivalence relation for sets