i suggest using `^xxxx` instead of `!xxxx` for inverse relationships to be closer to SPARQL inverse property paths

[tfrancart]

See https://www.w3.org/TR/sparql11-query/#pp-language

[davay42]

^p was initial syntax for inverse relations until MDLD 0.3 - but I had problems with unambiguous parsing ^^datatypes and ^p predicates and I decided to choose more unique marker so we don't need to count ^ symbols during parsing.