function interfaceSpacing( interfaces ) {
		var emptyLines;
		var elem;
		var end;
		var i;

		end = interfaces[ 0 ].end;
		if ( interfaces.length > 1 ) {
			for ( i = 1; i < interfaces.length; i++ ) {
				elem = interfaces[ i ];
				emptyLines = elem.start - end - 1;
				if ( emptyLines !== spacing ) {
					context.report( 'Interface documentation sections must be separated by `' + spacing + '` empty lines. Encountered: `'+ emptyLines+'`.', interfaces[ i ] );
				}
				end = elem.end;
			}
		}
	}
Checks that there are a specified number of empty lines between interfaces.

@private
@param {Array} interfaces - a list of interfaces