"""Helper function to convert an integer to its English text representation.

This function recursively converts an integer into its corresponding English text form. It handles numbers from 0 up to billions, including special cases for numbers less than 20, tens, hundreds, thousands, millions, and billions. The function uses predefined lists for ones and tens to map numbers to their word equivalents.

Args:
    num (int): The integer to be converted to text. Must be a non-negative integer.

Returns:
    str: The English text representation of the input integer.

Examples:
    >>> _int_to_text(5)
    'five'
    >>> _int_to_text(15)
    'fifteen'
    >>> _int_to_text(42)
    'forty two'
    >>> _int_to_text(123)
    'one hundred twenty three'
    >>> _int_to_text(2023)
    'two thousand twenty three'
    >>> _int_to_text(1000000)
    'one million'
"""