public FcgLevel child() {
		if (direction == IN_AND_OUT) {
			// undefined--this node goes in both directions
			throw new IllegalArgumentException(
				"To get the child of the source level you " + "must use the constructor directly");
		}

		return child(direction);
	}
Returns the child level of this level.  The child of a level has the same direction 
as this level, with a distance of one more than this level.

@return returns the child level of this level
@throws IllegalArgumentException if this is the source level, which is row 1
Ghidra/Features/GraphFunctionCalls/src/main/java/functioncalls/graph/FcgLevel.java