makunbound symbol ⇒  symbol
| symbol | a symbol | 
 (setf (symbol-value 'a) 1)
 (boundp 'a) ⇒  true
 a ⇒  1
 (makunbound 'a) ⇒  A
 (boundp 'a) ⇒  false
The value cell of symbol is modified.
Should signal an error of type type-error if symbol is not a symbol.