  We present a formal system for proving the partial correctness of a
single-pass instruction sequence as considered in program algebra by
decomposition into proofs of the partial correctness of segments of the
single-pass instruction sequence concerned. The system is similar to Hoare
logics, but takes into account that, by the presence of jump instructions,
segments of single-pass instruction sequences may have multiple entry points
and multiple exit points. It is intended to support a sound general
understanding of the issues with Hoare-like logics for low-level programming
languages.
