 Usage:  u = pswap(p, c)  
 
 Input:

  p                      n x 1 matrix of categories which have to be recoded 
                         into new ones or which be unchanged 
                         
  c                      l x 2 matrix of the old categories (first column) 
                         which have to be replaced by new category 
                         labels (second column) 
                         
                         
 Output:

  u                      n x 1 matrix containing the new categories of p 
                         
--------------------------------------------------------------
(C) MD*TECH Method and Data Technologies, 17.8.2000
