Problem : How to make an object follow another object in only 1 of the 3 dimensions (X, Y or Z) and leave the other two as is.
Solution (posted by Paul Tuersley on AE List September 17 2002)
otherposition = this_comp.layer("name of the other layer").position;
[position, position, otherposition];
, , and  relate to the x, y and z values contained in a three
dimensional property. So we're defining each one separately, but
putting them back together in the format [x, y, z].