HL7.Query.select

You're seeing just the function select, go back to HL7.Query module for more information.
Link to this function

select(msg, segment_selector)

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Specs

select(content_or_query_hl7(), binary()) :: t()

Selects or sub-selects segment groups in an HL7 message using a segment selector or filter function.

iex> import HL7.Query
iex> HL7.Examples.nist_immunization_hl7()
...> |> select("OBX")
...> |> select(fn q -> get_part(q, "1") != "1" end)
...> |> delete()
...> |> root()
...> |> get_segment_names()
["MSH", "PID", "ORC", "RXA", "RXR", "OBX", "ORC", "RXA", "ORC", "RXA", "RXR", "OBX"]