You should have read previous posts. Axiom does not understand "extend".
The following code does what you want. Just say
%axiom
)co sumlist.as
sumlist [1,2,3]
---BEGIN sumlist.as
#include "axiom"
SumPackage(R: Ring): with {
sumlist: List R -> R;
} == add {
sumlist (l: List R): R == {
s: R := 0;
for x in l repeat s:= s+x;
return s
}
}
---END sumlist.as
Have fun
Ralf
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