human
Solve the following python programming problem given the following code:
def _set_variables(self, cacheable) : 
    'Pull the variable names out of the provided expressions and separate into cacheable expressions and normal parameters. Those that are only stored in the cache, the parameters are stored in this object.' 
    def extract_vars(expr) : 
        return [e for e in expr.atoms () if (e.is_Symbol and (e not in vars))] 
self.cacheable = cacheable 
    self.variables = { 
        
} 
    vars = [] 
    for expression in self.expressions.values () : 
        vars += extract_vars (expression ['function']) 
self.cacheable_vars = [] 
    for var in cacheable : 
        self.variables [var] = [e for e in vars if (e.name.split ('_') [0] == var.lower ())] 
        self.cacheable_vars += self.variables [var] 
for var in cacheable : 
        if (not self.variables [var]) : 
            raise ValueError (((((('Variable ' + cacheable) + ' was specified as cacheable but is not in expression. Expected to find symbols of the form ') + var.lower ()) + '_0 to represent ') + var)) 
self.variables ['theta'] = sorted ([e for e in vars if (not (e in self.cacheable_vars))], key = (lambda e : e.name)) 

    
    

    Fix the buggy line: raise ValueError (((((('Variable ' + cacheable) + ' was specified as cacheable but is not in expression. Expected to find symbols of the form ') + var.lower ()) + '_0 to represent ') + var))
assistant
#Here is Python code to find symbols of the form ') + var.

raise ValueError (((((('Variable ' + var) + ' was specified as cacheable but is not in expression. Expected to find symbols of the form ') + var.lower ()) + '_0 to represent ') + var))