function onSVG( error, svg ) {
			var fopts;
			var fpath;
			var label;

			if ( error ) {
				debug( 'Error encountered when attempting to create an SVG. File: %s. : %s', file.path, error.message );
				return done( error );
			}
			label = LABEL.exec( nodes[ idx ].value );
			if ( label === null ) {
				debug( 'Invalid node: %s', nodes[ idx ].value );
				error = new Error( format( 'invalid node. Equation comments must have valid labels. Node: `%s`.', nodes[ idx ].value ) );
				return done( error );
			}
			label = label[ 1 ];
			debug( 'Equation label: %s', label );

			fpath = join( opts.dir, opts.prefix+label+'.svg' );
			fpath = resolve( file.dirname, fpath );
			debug( 'Absolute filepath: %s', fpath );

			fopts = {
				'encoding': 'utf8'
			};

			debug( 'Writing SVG to file...' );
			writeFile( fpath, svg, fopts, onWrite );
		}
Callback invoked upon creating an SVG.

@private
@param {(Error|null)} error - error object
@param {string} svg - SVG string
@returns {void}