code: function findDA (source, lineNumber) {
    for (var i = 0; i < source.length; i++) {
      var da = source[i]
      if (da.lineNumber === lineNumber) {
        return da
      }
    }
    return null
  }
docs: Find an existing DA record
  
  @param {DA[]}     source
  @param {number} lineNumber
  
  @returns {DA|null}