Get key set stored with a confrontation
keyset(x)# S4 method for confrontation
keyset(x)
an object of class confrontation
If a confrontation is created with the key= option
set, this function returns the key set, otherwise NULL
Other confrontation-methods: [,expressionset-method,
as.data.frame,confrontation-method,
confrontation-class,
confront, errors,
event,
length,expressionset-method,
values