Gobra does not consider `shStructDefault` to be terminating

[jcp19]

... as such, the following fails to verify

package main

type B struct {
	f int
}

//@ decreases
//@ pure
func (b *B)isNil() bool {
    return b == nil
}

Error message:

[info] Gobra has found 1 error(s) in package .. - main
[info] 	</Users/joao/Desktop/test_test.gobra:9:17> Pure function might not terminate. 
[info] Required termination condition might not hold.
[info] Gobra has found 1 error(s)
[info] Writing report to .gobra/stats.json