A graph 
that becomes disconnected when removing a suitable complete subgraph 
, called a vertex cut, is said
to be quasiseparable. The two simplest cases are those where 
is the null graph (which means
that 
is disconnected) or 
is the singleton graph
(which means that 
can be disconnected by removing one vertex, called articulation
or cut vertex). Under these circumstances, 
is called separable. A
forest is always separable, since every vertex of degree
at least two is an articulation vertex.
