What is interposition ( Programming)

Suppose a process is made up of several shared objects, and two shared objects, libX.so and libY.so, export the same symble xy(). under the traditional symbol search model any references to the symbol xy() will be bould to the first instance xy() that is found. So , if libX.so is loaded before libY.so then the instance of xy() within libX.so is used to satisfy all references. The instace of xy() within libX.so is said to interpose on the instance in libY.so