move lemmas about complex number 'i' to Complex.thy and Library/Inner_Product.thy
(* Author: Tobias Nipkow *)
theory Util imports Main
begin
subsection "Show sets of variables as lists"
text {* Sets may be infinite and are not always displayed by element
if computed as values. Lists do not have this problem.
We define a function @{text show} that converts a set of
variable names into a list. To keep things simple, we rely on
the convention that we only use single letter names.
*}
definition
letters :: "string list" where
"letters = map (\<lambda>c. [c]) chars"
definition
"show" :: "string set \<Rightarrow> string list" where
"show S = filter (\<lambda>x. x \<in> S) letters"
value "show {''x'', ''z''}"
end