code: def get(self, r):
          
          if r is None:
              return None
  
          if r.lower() == '(sp)' and self.stack:
              return self.stack[-1]
  
          if r[:1] == '(':
              return self.mem[r[1:-1]]
  
          r = r.lower()
          if is_number(r):
              return str(valnum(r))
  
          if not is_register(r):
              return None
  
          return self.regs[r]
docs: Returns precomputed value of the given expression