code: function generateSourceMappings(removedNodes) {
    var mappings = '';
    if (!removedNodes || removedNodes.length === '') {
      return mappings;
    }
  
    var end = {line: 1, column: 0};
  
    for (var i = 0; i < removedNodes.length; i++) {
      var start = removedNodes[i].loc.start;
      var lineDiff = start.line - end.line;
      var columnDiff = start.column - end.column;
      if (lineDiff) {
        for (var l = 0; l !== lineDiff; l++) {
          mappings += ';';
        }
        mappings += vlq.encode([start.column, 0, lineDiff, columnDiff]);
      } else if (columnDiff) {
        if (i) {
          mappings += ',';
        }
        mappings += vlq.encode([columnDiff, 0, lineDiff, columnDiff]);
      }
  
      end = removedNodes[i].loc.end;
      mappings += ',';
      mappings += vlq.encode([
        0,
        0,
        end.line - start.line,
        end.column - start.column,
      ]);
    }
  
    return mappings;
  }
docs: Generate a source map when *removing* nodes rather than replacing them with spaces.