  Structured natural languages provide a trade space between ambiguous natural
languages that make up most written requirements and mathematical formal
specifications such as Linear Temporal Logic. FRETish is a structured natural
language for the elicitation of system requirements developed at NASA. The
related open-source tool Fret provides support for translating FRETish
requirements into temporal logic formulas that can be input to several
verification and analysis tools. In the context of safety-critical systems, it
is crucial to ensure that a generated formula captures the semantics of the
corresponding FRETish requirement precisely. This paper presents a rigorous
formalization of the FRETish language including a new denotational semantics
and a proof of semantic equivalence between FRETish specifications and their
temporal logic counterparts computed by Fret. The complete formalization and
the proof have been developed in the Prototype Verification System (PVS)
theorem prover.
