
 The Disjoint Sum 
  Given two sets, A and B, we define their disjoint sum, A+B, to be the collection of symbols of form (a,) with a in A or of form (,b) with b in B. If A and B are ordered, we can extend their orders to A+B by deeming every (a,) to come before every (,b). 
  their intersection is empty), which is where the disjoint sum (or disjoint union) gets its name. 
  It should be noted that the standard embeddings of A and B in A+B compose with this to yield f and g as appropriate: in this sense, A+B is a minimal set via embeddings of A and B in which one may factorise any pair of functions, as above. 
 www.chaos.org.uk /~eddy/math/disjoint.html (603 words) 
