A vector of bits represented by a signed bigint. The most-significant bit is always normalised to 1, even when the underlying unsigned value would start with 0. When the unsigned bits already have their top bit set the stored value remains positive; otherwise the value is negated after toggling the leading bit to 1. This guarantees a unique representation for every combination of length and data.

• The sign bit acts as the stop bit, marking the boundary of the vector.
• The length is the count of meaningful bits, computed from the absolute value of the stored bigint.
• An empty vector is represented by 0n.