abbreviations for identical repeatable Elements

Allow lists in certain run time attributes. These lists are separated by whitespace.
The run time
<run_time>
<month month="2">
<monthdays>
<weekday day="monday">
<period single_start="10:10" />
</weekday>
<weekday day="tuesday">
<period single_start="10:10" />
</weekday>
</monthdays>
</month>
</run_time>

may then be written as
<run_time>
<month month="2">
<monthdays>
<weekday day="monday tuesday">
<period single_start="10:10" />
</weekday>
</monthdays>
</month>
</run_time>

These list should be enabled for these attributes:
Element <day> attribute "day"
Element <month> attribute "month"