Ada compilers are allowed to supplement the language-defined pragmas, and these are a potential source of non-portability. All GNAT-defined pragmas are described in the Implementation Defined Pragmas chapter of the GNAT Reference Manual, and these include several that are specifically intended to correspond to other vendors' Ada 83 pragmas. For migrating from VADS, the pragma Use_VADS_Size may be useful. For compatibility with HP Ada 83, GNAT supplies the pragmas Extend_System, Ident, Inline_Generic, Interface_Name, Passive, Suppress_All, and Volatile. Other relevant pragmas include External and Link_With. Some vendor-specific Ada 83 pragmas (Share_Generic, Subtitle, and Title) are recognized, thus avoiding compiler rejection of units that contain such pragmas; they are not relevant in a GNAT context and hence are not otherwise implemented.