Expr: An integrable function of one or more variables (limited only by memory). 

LOW: Optional constant of integration (or lower limit of integration if HIGH is included).

HIGH: Optional upper limit of integration.

Return value:
An EXPR, the iterated integral. 

Examples:
1. ii(x+y, {x, y})
   x²/2+xy+y

2. ii(x+y, {{x,a}, {y,b}}) or ii(x+y, [x,a;y,b])
   x²y/2+xy²/2+ay+b

3. ii(x+y, {{x,0,1}, {y,1,2}}) or ii(x+y, [x,0,1;y,1,2])
   2