Small enough to read in a sitting.

let var fn if else for in match known abstain know grounded validate use return

program   = { item } ;
item      = fn_decl | let_stmt | stmt ;
fn_decl   = "fn" IDENT "(" [ params ] ")" "->" type [ "validate" IDENT ] block ;
type      = base [ "?" ] ;                      (* T? : known or abstained *)
base      = IDENT [ "<" type { "," type } ">" ] ;
stmt      = let_stmt | for_stmt | match_stmt | expr ;
know_expr = "know" expr [ "as" IDENT ] "=>" expr
            "else" "=>" ( expr | abstain ) ;
abstain   = "abstain" text ;
match_stmt= "match" expr "{" known_arm abstain_arm "}" ;   (* both arms, either order *)
known_arm = "known" IDENT "=>" expr ;
abstain_arm = "abstain" IDENT "=>" expr ;