create a Prop with an empty label and an actual value
create a Prop with a label, an actual value, an expected value, and a constraint on the actual value
create a Prop with a label, an actual value, and a matcher on the actual value
create a Prop with a label, an expected value, and a constraint
create a Prop with a label, an expected value, and a constraint
create a Prop with a label and an expected value
Companion object with factory methods