Sum and product

[mikldk]

With 1.1.1, sum(expression, variable, lower, upper) can make summation as in yacas (summing an expression by letting a variable taking values from lower to upper).

But a simple

x <- ysym("x")
vec <- c(2*x, x)
sum(vec)
prod(vec)

is not possible.

[mikldk]

Fixed in 900726b.