(eplain.info)User-defined environments

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7.11 User-defined environments
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Plain TeX does not provide "named" block structures, only the anonymous
'\begingroup' and '\endgroup' pair. The disadvantage of this is that
when there are several such groups and one is mismatched, it can be
difficult to find the error. Eplain provides a named block structure so
that if you forget an '\environment' or an '\endenvironment', you will
(probably) get an error message about it.

   For example:

     \def\itpar{
       \environment{@italicpar}
       \it\par
     }
     \def\enditpar{
       \par
       \endenvironment{@italicpar}%
     }

which could then be used to set italicized paragraphs:

     \itpar
     If I reprehend anything in this world, it is the use of my oracular
     tongue, and a nice derangement of epitaphs!
     \enditpar

   The above sort of environment allows nesting. But environments
shouldn't always be allowed to nest. Put the control sequence
'\checkenv' at the beginning of a macro that is going to define an
environment that should not be nested.