IBM's Internal Specification Language of the 1980s

Based on your description, you're likely referring to Vienna Development Method (VDM), which was one of IBM's most significant formal specification languages developed in the 1970s and used throughout the 1980s.

About VDM

VDM was developed at IBM's Vienna Laboratory (hence the name) and became one of the earliest formal methods for specifying computer systems. The specification language component, known as VDM-SL (Vienna Development Method Specification Language), has several characteristics that match your description:

• It was based on set theory and mathematical logic
• It provided a type-safe formal notation
• It was independent of implementation languages
• It included tools for checking specifications for consistency

VDM was particularly popular within IBM during the exact timeframe you mention, and was used for various system specifications before other notations like Z (which you mentioned adopting at Hursley) became more widespread.

What Happened to It

VDM actually survived beyond IBM's internal use. It was later standardized as ISO/IEC 13817-1 and evolved into VDM++, which added object-oriented concepts. Today, it's still used in certain formal methods communities, and there are modern tools like Overture and VDMTools that support it.

Many IBM formal methods practitioners eventually transitioned to Z notation (as your team did) or other specification languages like B, as formal methods research and practice evolved through the 1980s and 1990s.

Could it possibly have been VDM that you remember using before adopting Z notation?