let fresh global sigma x =
let base = Var.basename x in
let tau = Var.var_type x in
let k = try Smap.find base !global with Not_found -> 0 in
let v = Var.ident_named_var base k tau in
global := Smap.add base (succ k) !global ;
v , Vmap.add x (e_var v) sigma