Let's say I am writing a credit spread on ES in an IRA that allows futures. I write the short end and take a credit in my tax-deferred account and but the smaller long end in a taxable account. I eat up some SPAN in my IRA, as my only added downside to this, from what I can tell. If it is not allowed what if I sold the ES again and bought the requisite number of SPYs in my taxable account as the long part of the credit spread.