You're seeing just the function digits, go back to Ergo.NumericParsers module for more information.
Link to this function

digits(opts \\ [])

The digits parser matches a series of at least one digit and returns an enumeration of the digits.


iex> alias Ergo.Context
iex> import Ergo.NumericParsers
iex> context = Ergo.parse(digits(), "2345")
iex> assert %Context{status: :ok, ast: [2, 3, 4, 5]} = context