// 4,6,9,10, the Places entry for X will contain [S,4].
	Places []StackSlot

	// Combined count of stacks where this source is the leaf.
	Self int64

	// Color number to use for this source.
	// Colors with high numbers than supported may be treated as zero.
	Color int
}

// StackSlot identifies a particular StackSlot.
type StackSlot struct {
	Stack int // Index in StackSet.Stacks
	Pos   int // Index in Stack.Sources