func (f exprFlags) Addressable() bool     { return f&32 != 0 }
func (f exprFlags) Assignable() bool      { return f&64 != 0 }
func (f exprFlags) HasOk() bool           { return f&128 != 0 }
func (f exprFlags) IsRuntimeHelper() bool { return f&256 != 0 } // a runtime function called from transformed syntax

func (f *exprFlags) SetIsVoid()          { *f |= 1 }
func (f *exprFlags) SetIsType()          { *f |= 2 }