Eraser morphisms and membership problem in groups and monoids

We develop the theory of fragile words by introducing the concept of eraser morphism and extending the concept to more general contexts such as (free) inverse monoids. We characterize the image of the eraser morphism in the free group case, and show that it has decidable membership problem. We establish several algorithmic properties of the class of finite--above (inverse) monoids. We prove that the image of the eraser morphism in the free inverse monoid case (and more generally, in the finite--above case) has decidable membership problem, and relate its kernel to the free group fragile words.