The DCX$EXPAND_INIT routine initializes the context area for the
    expansion of data records.

    Call the DCX$EXPAND_INIT routine as the first step in the
    expansion (or restoration) of compressed data records to their
    original state.

    Before you call DCX$EXPAND_INIT, read the length of the
    compressed file from the compression/expansion function (the
    map). Invoke LIB$GET_VM to get the necessary amount of storage
    for the function. LIB$GET_VM returns the address of the first
    byte of the storage area.