+ boolean-type istype? if true exit then
+ number-type istype? if true exit then
+ character-type istype? if true exit then
+ string-type istype? if true exit then
+ nil-type istype? if true exit then
+
+ false
+;
+
+: tagged-list? ( obj tag-obj -- obj bool )
+ 2over
+ pair-type istype? false = if
+ 2drop 2drop false
+ else
+ car objeq?
+ then ;
+
+: quote? ( obj -- obj bool )
+ quote-symbol tagged-list? ;
+
+: quote-body ( quote-obj -- quote-body-obj )
+ cadr ;
+
+: variable? ( obj -- obj bool )
+ symbol-type istype? ;
+
+: definition? ( obj -- obj bool )
+ define-symbol tagged-list? ;
+
+: definition-var ( obj -- var )
+ cdr car ;
+
+: definition-val ( obj -- val )
+ cdr cdr car ;
+
+: assignment? ( obj -- obj bool )
+ set!-symbol tagged-list? ;
+
+: assignment-var ( obj -- var )
+ cdr car ;
+
+: assignment-val ( obj -- val )
+ cdr cdr car ;
+
+: eval-definition ( obj env -- res )
+ 2swap
+ 2over 2over ( env obj env obj )
+ definition-val 2swap ( env obj valexp env )
+ eval ( env obj val )
+
+ 2swap definition-var 2swap ( env var val )
+
+ 2rot ( var val env )
+ define-var
+
+ ok-symbol
+;
+
+: eval-assignment ( obj env -- res )
+ 2swap
+ 2over 2over ( env obj env obj )
+ assignment-val 2swap ( env obj valexp env )
+ eval ( env obj val )
+
+ 2swap assignment-var 2swap ( env var val )
+
+ 2rot ( var val env )
+ set-var
+
+ ok-symbol
+;
+
+: if? ( obj -- obj bool )
+ if-symbol tagged-list? ;
+
+: if-predicate ( ifobj -- pred )
+ cdr car ;
+
+: if-consequent ( ifobj -- conseq )
+ cdr cdr car ;
+
+: if-alternative ( ifobj -- alt|false )
+ cdr cdr cdr
+ 2dup nil objeq? if
+ 2drop false
+ else
+ car
+ then ;
+
+: false? ( boolobj -- boolean )
+ boolean-type istype? if
+ false boolean-type objeq?
+ else
+ 2drop false
+ then
+;
+
+: true? ( boolobj -- bool )
+ false? invert ;
+
+: application? ( obj -- obj bool)
+ pair-type istype? ;
+
+: operator ( obj -- operator )
+ car ;
+
+: operands ( obj -- operands )
+ cdr ;
+
+: nooperands? ( operands -- bool )
+ nil objeq? ;
+
+: first-operand ( operands -- operand )
+ car ;
+
+: rest-operands ( operands -- other-operands )
+ cdr ;
+
+: list-of-vals ( args env -- vals )
+ 2swap
+
+ 2dup nooperands? if
+ 2swap 2drop
+ else
+ 2over 2over first-operand 2swap eval
+ -2rot rest-operands 2swap recurse
+ cons
+ then