module Mark:
k
coloring a graph with integer marks.
The provided function is more efficient that the one provided by functor
Coloring.Make
above.Parameters: 

exception NoColoring
val coloring : Coloring.G.t > int > unit
coloring g k
colors the nodes of graph g
using k colors,
assigning the marks integer values between 1 and k.
raises NoColoring
when there is no possible coloring.
The graph marks may be partially set before starting; the meaning of initial values is as follows:
NoColoring
if g
cannot be k
colored.