Question: How to create a double underscore in a macbook air. I type G__[], the underscore disappears

How to create a double underscore in a macbook air.  

I type G__[], the underscore disappears even on 1-D math.  I am using maple 2019.1, Mojave 10.14.6

I am trying to follow an example on geodesics 

 

with(Physics);

Setup(mathematicalnotation = true);
Setup(coordinates = X);

g_[];

 

I cannot reproduce the example because I cannot even create the syntax.

 

 

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