<function>extractQuery</> to pass additional data to the

<function>consistent</> and <function>comparePartial</> methods.

To use it, <function>extractQuery</> must allocate

an array of <literal>*nkeys</>Pointers and store its address at

an array of <literal>*nkeys</>pointers and store its address at

<literal>*extra_data</>, then store whatever it wants to into the

individual pointers. The variable is initialized to <symbol>NULL</symbol> before

call, so this argument can simply be ignored by operator classes that