e(X) == local-state(x,y. x+y;0;X;e) 

latex

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clarification:

es-interface-sum{i:l}(es; X; e) == es-interface-local-state{i:l}(es; (x,y. x+y); 0; X; e) 

latex

Definitions	#$n, n+m, x.A(x), local-state(f;base;X;e)
FDL editor aliases	es-interface-sum