Field Lists
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:what: Field lists map field names to field bodies, like database
       records.  They are often part of an extension syntax.  They are
       an unambiguous variant of RFC 2822 fields.

:how arg1 arg2:

    The field marker is a colon, the field name, and a colon.

    The field body may contain one or more body elements, indented
    relative to the field marker.