Function avg() calculates the average value of a column of numbers. Only one row from the data stream will be kept after this function is called, and all the rest rows will be abandoned.
Arguments：
- Number <number>, auto-abstraction: UUID of NODE/EDGE, row index (starting from 0) of PATH/TABLE, or the first element of ARRAY
Returns：
- Avg value <number>
ATTR (Numeric value)
Example: find the outbound 1-step paths from Card CA001, and calculate the average value of the amount of money transferred
n({_id == "CA001"}).re({@transfer} as pay).n({@card})
return avg(pay.amount)