Create a combining matrix

## Arguments

- K
The original number of classes: the matrix will define a combining from K to (K-1) classes.

- l1
Label of one of the two classes to be combined.

- l2
Label of the other class to be combined.

## Value

If `z`

is a vector (length *K*) whose *k*th entry is the probability that an observation belongs to the *k*th class in a *K*-classes classification, then `combiM %*% z`

is the vector (length *K-1*) whose *k*th entry is the probability that the observation belongs to the *k*th class in the *K-1*-classes classification obtained by merging classes `l1`

and `l2`

in the initial classification.

## Author

J.-P. Baudry, A. E. Raftery, L. Scrucca