Constant KEY_LEAD 

pub const KEY_LEAD: i32 = _; // 3i32

Periods between deciding key k and its chain 0: one before its first op.

A key must be decided (real key or bubble) before its earliest op decodes, because decode reads the key’s KeyEntry::real flag. The earliest op has lag lag_range().0 (negative); deciding one period earlier than that is 1 − lag_range().0 periods before chain 0.