Make stitchedArcs a Set.

[martinfrances107]

I have a minor nudge.

var stitchedArcs = {}

Looking at the life-cycle of the object stitchedArcs

It has the characteristics of a set ( i.e. a [key,value] pairs where the value is only ever set to '1' )

I would like to make this semantically more precise.

By semantically I mean I want to, lock this down, to give assurances that the value is never accidentally incremented or set to a a value like as 'socks'.

I am about to submit a three line PR