Fast reverse iteration for BTSet and slices
datascript/datom call to create a datom
adding aggregate built in fns avg, median, variance and stddev
added count-distinct aggregate (closes #42)
min and max aggregates use comparator instead of default js < comparison
Update README.md
Fixed a bug when fn inside a query on empty relation returned non-empty result
Filtered databases support via filter
version 0.7.0