code: function newParam (columnName) {
      var i = 0
      // get placeholder name based on column name with counter
      // added if the same column name is used multiple times
      while (i < 99) {
          // use column name as base for param name
          var paramName = columnName+i
          // if param name is not in use then use it - caller will add
          // param to params map with value for substitution
          if (this.params[paramName] === undefined) {
              return paramName
          }
          // try next number
          i++
      }
      // if placeholder could not be found throw error
      throw new Error('max paramName exceeded '+columName)
  }
docs: @function newParam
  
  get param name from column name
  
  @params {string} columnName
  
  @returns {string}
  
  @throws {Error}