<lambda expression> --> (lambda <formals> <body>)
<quotation> --> '<datum> | (quote <datum>)
<assignment> --> (set! <variable> <expression>)
<conditional> --> (if <test> <consequent> <alternate>)
<definition> --> (define <variable> <expression>)
      | (define (<variable> <def formals>) <body>)
      | (begin <definition>*)