.sp
If both of the source objects have value data and no maps, the gate object 
must have value data, and it will be used to define the gating.
.sp
If both of the source objects have maps and no value data, the gate object 
must have a map, and it will be used to define the gating.
.sp
If either of the source objects have both value and map data, all objects with
maps, including the gate, will be mapped before processing.  
.sp
If the gating value is zero, the first source object's value will be 
transferred to the destination object.
