Returns the current output context as a string.
"data" | In top level data. This is always returned when called from
tag or container callbacks.
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"arg" | In an unquoted argument.
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"splice_arg" | In a splice argument.
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The return value can also be a single character string, in which
case the context is a quoted argument. The string contains the
starting quote character.
This function is typically only useful in entity callbacks, which
can be called both from text and argument values of different
sorts.