How to use the expression.Variable function in Expression

def is_variable(e):
    return isinstance(e,Variable)

rpn_stack.append((tok,val,0))
            elif tok == 'IDENTIFIER':
                
                paramOrVariable = self.symtable.get(val, None)
                if not paramOrVariable: raise SyntaxError("\"%s\"\n\tUnknown identifier \"%s\"." % (s, val))
                                
                # keep track of identifiers we've used (this is in the
                # original problem). since we introduce new variables, i 
                # don't want them "shown" at the end. this is just a separate 
                # data structure to keep track of variables referened in the 
                # original problem.
                self.used_syms.add(val)
                
                
                # keep track of minimal variable and param references (in the rewritten problem)
                if isinstance(paramOrVariable,Variable):
                    self.codegen.variables[val] = paramOrVariable
                else:
                    self.codegen.parameters[val] = paramOrVariable
                
                rpn_stack.append( (tok,paramOrVariable,0) )
            elif tok == 'MACRO':
                # functions have highest precedence
                is_function_call = True
                op_stack.append((tok,val,0))
                argcount_stack.append(0)                
            elif tok == 'COMMA':
                # same as semicolon. TODO: merge the two
                op = ""
                while op != 'LPAREN':                    
                    if op_stack:
                        op,sym,arg = op_stack[-1]   # peek at top

def expand(self, x):
        """Expands parameters and constants"""
        if isinstance(x,Variable):
            # add to varlist
            self.varlist[x.name] = x
            return x
        elif isinstance(x,Expression):
            return x
        else:
            if x.shape is MATRIX:
                raise Exception("Cannot expand %s since it is a matrix." % x.name)
                
            v = self.new_var(x.shape)
            m = v.name
                
            if isinstance(x,Constant):
                row = [(Eye(1),v), (Ones(1,x.value),)]
            elif isinstance(x,Parameter):
                row = [(Eye(m),v), (Vector(x),)]

def new_var(self,shape):
        """Creates a new, temporary variable"""
        name = 't' + str(self.varcount)
        self.varcount += 1
        v = Variable(name, shape)
        self.tmplist[name] = v
        return v

elif v in self.symtable:
                raise Exception("\"%(s)s\"\n\tThe name %(v)s is already in use for a parameter / variable." % locals())
        
            shape = 'SCALAR'
            # optionally parse shape
            if toks:
                shape, tmp  = toks.popleft()
                if shape != 'VECTOR' and shape != 'SCALAR':
                    raise SyntaxError("\"%(s)s\"\n\tExpected a VECTOR or SCALAR shape, but got %(tmp)s instead." % locals())
        
                # if any remaining
                if toks:
                    t, tmp = toks.popleft()
                    raise SyntaxError("\"%(s)s\"\n\tUnexpected ending for variables with %(t)s token %(tmp)s." % locals())
            
            self.symtable[v] = Variable(v,build_shape[shape](v))
        else:
            raise SyntaxError("\"%(s)s\"\n\tNo variable name provided." % locals())

t, v = toks.popleft()
        if t is not "IDENTIFIER":
            raise SyntaxError("\"%(s)s\"\n\tExpected an identifier, but got %(t)s with value %(v)s instead." % locals())
        elif v in self.symtable:
            raise Exception("\"%(s)s\"\n\tThe name %(v)s is already in use for a parameter / variable." % locals())
            
        shape, tmp  = toks.popleft()
        if shape is not "VECTOR" and shape is not "SCALAR":
            raise SyntaxError("\"%(s)s\"\n\tExpected a VECTOR or SCALAR shape, but got %(tmp)s instead." % locals())
        
        # if any remaining
        if toks:
            t, tmp = toks.popleft()
            raise SyntaxError("\"%(s)s\"\n\tUnexpected ending for variables with %(t)s token %(tmp)s." % locals())
                
        self.symtable[v] = Variable(shape)