An integral current is a rectifiable current with integer multiplicities whose boundary
is also rectifiable with integer multiplicities. In Euclidean space it describes an oriented rectifiable
set counted with integer weights, together with the
induced boundary. For example, integration over an oriented
ball gives an integral current whose boundary
is integration over its sphere. The mass 
measures the total volume counted
with absolute multiplicity. The notion also extends to metric
spaces, where it is defined using Lipschitz test data.
A metric space is proper when every closed bounded set is compact.
