I am currently working on double cell operations built into the zeptoforth kernel (so they will get a performance boost over if they were implemented in Forth), which should soon make their way into a 0.2.0 release. I must give credit to Matthias Koch, as I am heavily borrowing upon his 64-bit mathematical operations as implemented in Mecrisp-Stellaris. Release 0.2.0 will also contain implementations of DEFER, DEFER!, and DEFER@; note that DEFER! differs from its ANS counterpart in that a deferred word in flash will only be writable once (and executing a deferred word before it is written will result in a crash rather than an abort), whereas ANS DEFER! as I understand it allows multiple writes.