Routine Name:	getsynindex

Description:	getsynindex is used to find the index of synapses between
		particular presynaptic and postsynaptic elements.  

Usage:		getsynindex presynaptic-element postsynaptic-element
                        [-number n]

                presynaptic-element  The element sending SPIKE messages to
                        postsynaptic targets.

		postsynaptic-element The synchan or derived element
		       receiving the SPIKE message from the presynaptic
		       element.

		-number n  If there is more than one synapse between the
			   given pre- and postsynaptic elements, this option
			   will return the index of the nth such synapse.
			   This option will rarely be necessary, since
			   usually there is at most one synapse between a
			   given presynaptic and postsynaptic element.  

Example:	int index = {getsynindex /input[0]/spike /cell/soma/Ex_channel}
		if (index >= 0)
		    setfield /cell/soma/Ex_channel synapse[{index}].weight 10.0
		end

Notes:		If the desired synapse does not exist a value of -1 is
                returned.

See also:	getsyncount, getsynsrc, getsyndest, BoG chapter 17

