Both of these work fine, the second being more compact. Thanks.
After floundering around, however, I believe the easiest way to find the positions(s) of single element lists is to use pos=Position[ mat, {}]. For double element lists (and so on) use Position[mat, {,_}].
After you have found the position in the form {i,j} , you can recapture that element of matrix mat with Extract[ mat, pos].
KH
