iftex – Am I running under pdfTeX, X∃TeX or LuaTeX?

The pack­age, which works both for Plain TeX and for LaTeX, de­fines the \ifPDFTeX, \ifX∃TeX, and \ifLuaTeX con­di­tion­als for test­ing which en­gine is be­ing used for type­set­ting.
The pack­age also pro­vides the \Re­quirePDFTeX, \Re­quireX∃TeX, and \Re­quireLuaTeX com­mands which throw an er­ror if pdfTeX, X∃TeX or LuaTeX (re­spec­tively) is not the en­gine in use.