Restructure Q-sort responses to proximity matrix in SPSS

Problem

I have an SPSS data file with responses from a Q-sort study and wish to reorganize the data into a matrix of co-occurrence frequencies for analysis by CLUSTER or ALSCAL. Each respondent was given 21 stimuli and asked to sort them into piles that represented similar groups. Each respondent is a case in my data file and variables ITEM1 to ITEM21 identify the piles into which the respective stimuli were placed. For example, if a respondent placed the 5th stimulus into his/her 8th pile, item5 would hold an 8 for that respondent. My target data structure is a 21x21 matrix in which the element in row i and column j indicates the number of respondents who placed items i and j in the same pile. The numerical ordering of piles is irrelevant (although this is not necessarily true for all Q-sort studies). Items i and j may be grouped together in the 4th pile for one respondent; the 9th pile, for a different respondent. Both observations would add to the (i,j) and (j,i) elements of the matrix.