x { subtract i

C subtract a series of numbers starting from 1
C e.g. subtract 10 gives:
C 1 -2 -3 -4 -5 -6 -7 -8 -9 -10
C evaluated in left to right order
C stored in n

a { -I(i)
n { 1, a[1+I(i-1)]
'the series is ',NJ n
x { +/ n