:: (Uniquable k, Uniquable cls, Uniquable color, Eq color, Eq cls, Ord k, Outputable k, Outputable cls, Outputable color)  
=> Bool  whether to do iterative coalescing

> Int  how many times we've tried to color this graph so far.

> UniqFM (UniqSet color)  map of (node class > set of colors available for this class).

> Triv k cls color  fn to decide whether a node is trivially colorable.

> Graph k cls color > k  fn to choose a node to potentially leave uncolored if nothing is trivially colorable.

> Graph k cls color  the graph to color.

> (Graph k cls color, UniqSet k, UniqFM k)  
Try to color a graph with this set of colors.
Uses Chaitin's algorithm to color the graph.
The graph is scanned for nodes which are deamed 'trivially colorable'. These nodes
are pushed onto a stack and removed from the graph.
Once this process is complete the graph can be colored by removing nodes from
the stack (ie in reverse order) and assigning them colors different to their neighbors.
