[isabelle] how to rewrite rule_tac x="A" in exI as tactic?


I was hoping that someone of you could give me a little help with tactics.

I try to rewrite an apply-style proof into a tactic, for future

my goal:
rewrite apply(rule_tac x="A" in exI) as a tactic

my attempt:
val tac = let x = @{term "A"} in rtac @{thm exI} 1 end

my problem:
the bound variable x is represented in another way, i.e. I'm substituting
nothing with the let-in command.

my questions:
1) could somebody tell me, how it will work
2) how can i display the internal representation of a theorem (on ML-level,
so that i can see which term instead of x i need to substitute)

thanks in advance

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