lisp interpreter in python

I'm curious how a part of Peter Norvig's Lisp interpreter works. One can define functions in this Lisp interpreter...how does this work? I'm a beginner, and just would like a simple explanation.

There's one part that might be related where it says

elif x[0] =开发者_开发百科= 'define':         # (define var exp)
        (_, var, exp) = x

http://norvig.com/lispy.html

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In this case, x[0] is define, x[1] is the variable name, and x[2] is the expression. So, in Python, _, var, exp = x is a "destructuring assignment", which destructures the array x into its constituent elements, and assigns them to the variables on the left-hand side.