+ local func = getfenv()[self.name]
+ assert(func ~= nil,
+ 'Cannot resolve function "' .. self.name .. '". Is it misspelled or local?')
+
+ assert(type(func) == "function",
+ 'The symbol "' .. self.name .. '" does not refer to a function but data ' ..
+ 'of type "' .. type(func) .. '".')
+