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Conditionally repeated block. Its arguments are evaluated (see section 3.6.12 EXPR - Evaluate and emit an Expression)
and as long as the result is non-zero or a string with one or more bytes,
then the condition is true and the text from that point
until a matched
ENDWHILE is emitted.
[+WHILE <full-expression> +] emit things that are for the true condition[+ ENDWHILE +]
<full-expression> may be any expression described in the
EXPR expression function, including the use of apply-codes
and value-names. If the expression yields an empty string, it
is interpreted as false.