A condition is formed by:
<tag> <op> <value>
<tag> is the normal yambar tag. <op> is one of '==', '!=', '<', '<=', '>', or
'>='. <value> is what you wish to compare it to.
'boolean' tags must be used directly. They falsehood is matched with '~':
<tag>
~<tag>
Finally, to match an empty string, one must use ' "" ':
<tag> <op> ""
When set, river tags and seats’ view titles apply to the output yambar
is on, only.
The default is disabled, which implements the old behavior, where
river tags and seats’ view titles represent the union of all
outputs.