  An interval in a combinatorial structure S is a set I of points which relate
to every point from S I in the same way. A structure is simple if it has no
proper intervals. Every combinatorial structure can be expressed as an
inflation of a simple structure by structures of smaller sizes -- this is
called the substitution (or modular) decomposition. In this paper we prove
several results of the following type: An arbitrary structure S of size n
belonging to a class C can be embedded into a simple structure from C by adding
at most f(n) elements. We prove such results when C is the class of all
tournaments, graphs, permutations, posets, digraphs, oriented graphs and
general relational structures containing a relation of arity greater than 2.
The function f(n) in these cases is 2, \lceil log_2(n+1)\rceil, \lceil
(n+1)/2\rceil, \lceil (n+1)/2\rceil, \lceil log_4(n+1)\rceil, \lceil
\log_3(n+1)\rceil and 1, respectively. In each case these bounds are best
possible.
