Bus comparison is supported in Definitions but not in Guarantees

hi! Not sure it is a bug or a feature: you can write `a==b` for two buses `a` and `b` in section `DEFINITIONS` but not in `GUARANTEES`:

```
INFO
{
  TITLE:       ""
  DESCRIPTION: ""
  SEMANTICS:   Mealy
  TARGET:      Mealy
}
GLOBAL
{
  DEFINITIONS { gua(a,b) = a==b; } // OK
}
MAIN
{
  INPUTS { a[10]; b[10]; }
  OUTPUTS { out; }
  GUARANTEES { G a==b; } // not OK
}
```