https://kalyan-city.blogspot.com/2012/09/functions-of-business-internal-external.html

https://stackoverflow.com/questions/20552658/where-is-the-definition-of-external-functions-in-a-header-file

https://stackoverflow.com/questions/1041866/what-is-the-effect-of-extern-c-in-c

https://stackoverflow.com/questions/20552658/where-is-the-definition-of-external-functions-in-a-header-file/20552885

https://docs.mql4.com/basis/function/extfunctions

https://docs.moodle.org/dev/External_functions_API

https://docs.moodle.org/dev/External_services_description

https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/functions/external-functions

https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-external-events

http://wiki.heroengine.com/wiki/External_functions