An environment is a sequence of frames.

eval define: bind a symbol to a value in a frame (environment)

eval lambda: create a procedure object (two circles)
	     has a pointer to env where lambda was evaluated
	     has list of parameters
	     has the body of the procedure

eval a call: create a new frame (a square) pointing to procedure's environment
	     bind parameters to the values of the arguments
	     evaluate the body in this (new) environment


To evaluate a combination: (same as before)
   evaluate the subexpressions
   apply the value of the first sub-expr to the remaining values