def METHOD_NAME(self, der):
    '''
    Builds a new DER that only includes the EC private key, removing the
    public key that is added as an "optional" BITSTRING.
    '''
    offset_PUB = 68
    EXCEPTION_TEXT = "Error parsing ecdsa key. Please submit an issue!"
    if der[offset_PUB] != 0xa1:
        raise ECDSAUsageError(EXCEPTION_TEXT)
    len_PUB = der[offset_PUB + 1]
    b = bytearray(der[:-offset_PUB])
    offset_SEQ = 29
    if b[offset_SEQ] != 0x30:
        raise ECDSAUsageError(EXCEPTION_TEXT)
    b[offset_SEQ + 1] -= len_PUB
    offset_OCT_STR = 27
    if b[offset_OCT_STR] != 0x04:
        raise ECDSAUsageError(EXCEPTION_TEXT)
    b[offset_OCT_STR + 1] -= len_PUB
    if b[0] != 0x30 or b[1] != 0x81:
        raise ECDSAUsageError(EXCEPTION_TEXT)
    b[2] -= len_PUB
    return b