Underload interpreter

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Problem

Write an interpreter for the programming language Underload; you need not implement quoting with ", because no other Underload implemntation does so either. . The spec for the language is at http://esolang.org/wiki/Underload, but in short, it's a stack-based language with the current commands:

(anything with matched parens, the parens are part of the command) push whatever is between the outermostparens on the stack

: duplicate the top of stack

! pop the top of stack

^ evaluate the top of stack as an Underload program, popping it

~ swap the top two stack elements

* concat the top stack element to the end of the second stack element

S output the top stack element, popping it

a enclose the top stack element in parents

Underload is a good language for writing quines in, as it happens. It is also Turing-complete.

You do not need to handle errors; only completely perfect Underload code need be evaluated correctly.

Options

now post-mortem time, all source codes will be revealed

Sample input:_

(:aSS):aSS

Sample output:

(:aSS):aSS

Sample input:_

(:::::):(:((^:()~((:)*~^)a~*^!!()~^))~*()~^^)~(^a(*~^)*a~*()~^!()~^)a~**^!!^S

Sample output:

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Sample input:_

(x)(::**)(::**)^^S

Sample output:

xxxxxxxxxxxxxxxxxxxxxxxxxxx

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