Issue with undefined function

I am having a problem calling a function. There seems to be two kinds of modules in my project. A) module that has functions that extend ci_model . These functions allow you to use this->. Typical CI stuff. B) module that is just straight php code. These functions don't extend the ci_model. 

I have a need to call a function in a A module from a B module. This won't work and I get an undefined function error when I run the call. What must I add to module B to tell it to look in Module A for the missing function?

I hope I explained this ok?

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