public class CompositePebble
A combination of pebbles from different sources. The mergePebbles methods all obey the following rules: 1) The merged
pebble combines all of the PurePebbles of both and the multiplicity of the minimum of the multiplicities of the two
merged pebbles. 2) If a and b have a common PurePebble: a) It is merged into a single PurePebble, with mass set to
the sum of the masses of the parts. b) If the mass is greater than the number of pebbles on the source, it is
illegal, and mergePebble returns null.
Returns true if and only if 'other' contains every PurePebble this object does, and the mass of each PurePebble
(that is, from each source) of 'other' is at least the mass in this object. Thus, it returns false if this object
contains a PurePebble that 'other' does not, or if this one contains a PurePebble from a given source with more
mass than than the corresponding one in 'other'. If this and other are equal, it returns true.