text: function Problem(width) {
    let _width = width;
    let _rows = [];
    
    this.width = function width() {
      return _width;
    };
    
    this.rows = function rows() {
      return _rows.concat([]);
    };
    
    this.addRow = function addRow(rowArg) {
      let row = rowArg.concat([]);
      row.sort();
      
      for (let i = 0; i < row.length; i += 1) {
        if (row[i] >= _width) {
          throw new Error('Column out of range in row');
        }
        if (i > 0 && row[i - 1] === row[i]) {
          throw new Error('Duplicate columns in row');
        }
      }
      
      _rows.push(row);
    }
  }
  
  module.exports = Problem;