code: function sum(transform) {
    let sum = 0;
    let seq = typeof transform === 'function' ? this.select(transform) : this;
  
    for (let item of seq) {
      sum += Number.parseFloat(item);
    }
  
    return sum;
  }
docs: Computes the sum of a sequence.
  
  @param transform The transform function called per interaction.
  @return The sum of the values in the sequence.