I have class `mSkeleton` with some methods:

```
//mSkeleton Class
mSkeleton::mSkeleton( const double& M, const int& N){ 
mass = M;
nph = N;
}

double mSkeleton::getNph(){
  return nph;
}
...

```

And I have another class `mizer` with some methods:

```
void mizer::addMassSkeleton( const mSkeleton& mass ){
  masses.push_back( mass );
  count_external_phs += mass.getNph();
}

```

When I compile it I get the error:

```
mizer.C:46:24: error: member function 'getNph' not viable: 'this' argument has type
const mSkeleton', but function is not marked const
count_external_phs += mass.getNph();

```

How to fix this?