This function accepts a span (of type CDM_Span) and returns the start of
this span. Each span object is a set of exactly two integers, named as
"start" and "end".
Return Value:
This function returns the start value of the provided span object. If
the provided object is not a valid span object or some other error
occurs, CDM_GetStart will return -1 and an error message describing the
error will be placed at the current Tcl interpreter (CDM_Interp).