The function subst makes a new tree structure with a specified substitution. subst is standard in Common Lisp.

(subst new old tree ) substitutes new for old in tree.

(subst 'jones 'name '(dear mr name))

   =  (dear mr jones)

(subst 5.0 'radius
       '(* 3.14159 (expt radius 2)))

   =  (* 3.14159 (expt 5.0 2))

(subst 'socrates 'x
       '(if (human x) (mortal x)))

   =  (if (human socrates) (mortal socrates))

(subst 'lollipop 'rose
       '(rose is a rose is a rose))

   =  (lollipop is a lollipop is a lollipop)

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