Function sum() sums up the numbers in an alias. Only one row from the alias 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：
- Sum <number>
Comman Usage
Example: find the outbound 1-step transfer paths from Card CA001, return the sum of the transfer amounts
n({_id == "CA001"}).re({@transfer} as pay).n({@card})
return sum(pay.amount)