Undefined function, even though imported

[Loopback]

Hi!

Let me begin by saying, I'm sorry if this is caused of some obvious 
error but since I am new to D, I am not aware of all the tricks and 
treats it offers.

I am working with the WindowsAPI binding at dsource.org (though I do not 
believe this is related to the binding itself). However, in my code I 
call the function LOWORD (win32 specific). This function is defined in 
the win32.windef module. Although it is defined there (without any 
encapsulations in version statements or anything similar) I receive this 
error:

Error 42: Symbol Undefined _D5win326windef6LOWORDFkZt (LOWORD undefined)

To solve this I had to copy-paste the exact function directly into my 
own file/module instead. So my question is the following; why do I have 
to copy and paste the function directly in my code when it is clearly 
defined in one of the files I import?

Windef.d attached
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