Question: Use of the 'sum' command.

Does anyone know how the sum command works? If I set

f:=proc(x) if x<=1 then 1 else 2

then compute


I get an error, as it tries to check "i<=0", for the symbol 'i'. Curiously if the comparison in 'f' is "x=1" it goes through fine. Another place this comes up is using "op" on lists. If I set


then compute

sum(foldl((x,y)->x*vars[y],1,op(z[i])),i =1..3)

it doesn't give the desired answer. When i=1 for example, I think it computes op(z[i]) as op(z[i])->i->1, rather than op(z[i])->op(z[1])->op([1,2])->1,2.

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