function visitor( node, index, parent ) {
		var label;
		var raw;
		var err;

		if ( EQN_START.test( node.value ) === true ) {
			debug( 'Found an equation...' );

			label = LABEL.exec( node.value );
			if ( label === null ) {
				debug( 'Invalid node: %s', node.value );
				err = new Error( format( 'invalid node. Equation comments must have a valid label. Node: `%s`.', node.value ) );
				return done( err );
			}
			label = label[ 1 ];
			debug( 'Label: %s', label );

			raw = RAW.exec( node.value );
			if ( raw === null ) {
				debug( 'Invalid node: %s', node.value );
				err = new Error( format( 'invalid node. Equation comments must have valid equation text. Node: `%s`.', node.value ) );
				return done( err );
			}
			raw = raw[ 1 ];
			debug( 'Raw equation: %s', raw );

			equations.push({
				'parent': parent,
				'index': index,
				'label': label,
				'raw': raw
			});
		}
	}
Callback invoked upon finding a matching node which searches for Markdown equations.

@private
@param {Node} node - reference node
@param {number} index - position of `node` in `parent`
@param {Node} parent - parent of `node`
@returns {void}