def _DECPERIOD(x, p=1):
    """
    Forces an increase `p` in the minperiod of object `x`.

    Example: `ta-lib` calculates `obv` already when the period is `1`,
    discarding the needed "close" to "previous close" comparison. The only way
    to take this into account is to decrease the delivery period of the
    comparison by 1 to start the calculation before (and using a fixed
    criterion as to what to do in the absence of a valid close to close
    comparison)
    """
    x._minperiod -= p
