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win64?
winapi

5.10 Windows API Helpers

 (require ffi/winapi)

Indicates whether the current platform is 64-bit Windows: #t if so, #f otherwise.

winapi : (or/c 'stdcall 'default)
Suitable for use as an ABI specification for a Windows API function: 'stdcall on 32-bit Windows, 'default on 64-bit Windows or any other platform.