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Anything Excel - but specific in-depth (and unusual) knowledge in Excel VBA, and how to extract from it functionality that Bill Gates' team did not intentionally cater for (though they should have).
Old-school (mid-1980s) Machine-Code and later Assembler programmer working Z-80, 6502 and 8086 processors; as a commercial games programmer, I authored Interdictor Pilot (the very first real-time flight-and-fight space combat simulator) on TRS-80, C-64 and CPC-464 platforms. Also authored the games program Kamikaze-Khazi. Have been working in Excel VBA for 9 years, with a leaning towards business applications. I have a current commercial, specialist Excel VBA application licensed to clients requiring an environment that delivers integrated requirements engineering and assessment of competing bids on large scale procurement projects (typically £1M - £400M); it is licensed at £10k. It is quite large (~ 6 mB) and is very powerful - it has Excel performing feats beyond its design spec and integrates with MS Word to produce a wide variety of interactive print-ready outputs.
UK Masters-degree level Aerosystems
UK Ministry of Defence (DPA, DLO and DE&S), BAE Systems, Northrop Grumman, Lockheed Martin, ULTRA Defence Systems, Offshore International, Signature Industries, Fernau Ltd, Rockwell Collins (UK)...