An environment is a sequence of frames.

eval define: bind a symbol to a value in an 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) 
             square points to procedure's environment
             bind parameters to the values of the arguments
             evaluate the body in this (new) environment

eval set!: Change the value bound to var in the closest frame of the
           environment

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