Parametric entities

only inside of DTD, somehow similar to "macros" in pg. languages

suitable eg. for declations of attribute lists (if long and multiply used)

see DTD for HTML 4.01 - http://www.w3.org/TR/html4/sgml/dtd.html

definition of a parametric entity is eg. <!ENTITY % heading "H1|H2|H3|H4|H5|H6">