Preamble: Every mapping function is identified by the subscripted name that
follows its right-hand bold square bracket. For a few mapping functions,
the subscript also contains a parameter, which is usually a metavariable,
e.g. 'Type1' in
    [[ ... ]]_FunctionArgument(Type1)

Currently, when a metavariable occurs as a mapping's subscript-parameter,
it is rendered abnormally (relative to the rendering of metavariables
everywhere else):
-- It is shown in an upright font (vs slanted/italic).
-- If it itself has a 'subscript', that subscript appears on the same
   baseline (vs being on a lowered baseline).
Both effects can be seen in 4.1.5 [Function Calls] / Normalization / rule 1

Instead, metavariables appearing in mapping function subscripts should be
rendered as they are elsewhere.