Building systems

The main system builder is Operators. Give it particle masses and, when convenient, charges:

ops = Operators([1.0e15, 1.0], [+1.0, -1.0])
ops += "Kinetic"
ops += "Coulomb"

ops += term builds a new Operators containing the extra term and rebinds ops to it; push!(ops, term) adds the term to the existing object in place. Any earlier reference to ops keeps its old term list after +=, but sees the new term after push!.

Masses and charges are in atomic units. A very large mass such as 1.0e15 is the usual fixed-nucleus approximation.