Pfx.eui64
You're seeing just the function
eui64, go back to Pfx module for more information.
Specs
Create a modified EUI-64 out of eui48 (an EUI-48 address).
Despite rfc7217, modified EUI-64's are still used in the wild.
This flips the 7-th bit and inserts 0xFFFE in the middle.
Example
iex> eui64("0088.8888.8888")
"02-88-88-FF-FE-88-88-88"
iex> eui64("0288.8888.8888")
"00-88-88-FF-FE-88-88-88"
iex> from_mac({0x00, 0x88, 0x88, 0x88, 0x88, 0x88}) |> eui64() |> digits(8)
{{0x02, 0x88, 0x88, 0xFF, 0xFE, 0x88, 0x88, 0x88}, 64}