Called by the processor when it encounters an xsl:include, xsl:import, or document() function.

Parameters:

href		An href attribute, which may be relative or absolute.
base		The base URI against which the first argument will be made absolute if the absolute URI is required.

Returns:  A Source object, or null if the href cannot be resolved, and the processor should try to resolve the URI itself.

Exceptions:

TransformerException

if an error occurs when trying to resolve the URI.