Function def_environment 

pub fn def_environment(
    name: String,
    paramlist: Option<Parameters>,
    compiled_replacement: Option<ReplacementClosure>,
    options: ConstructorOptions,
)

Defines an Environment that generates a specific XML fragment.

compiled_replacement is of the same form as for DefConstructor, but will generally include reference to the #body property. Upon encountering a \begin{env}: the mode is switched, if needed, else a new group is opened; then the environment name is noted; the beforeDigest hook is run. Then the Whatsit representing the begin command (but ultimately the whole environment) is created and the after_digest_begin hook is run. Next, the body will be digested and collected until the balancing \end{env}. Then, any after_digest hook is run, the environment is ended, finally the mode is ended or the group is closed. The body and \end{env} whatsit are added to the \begin{env}’s whatsit as body and trailer, respectively.