vec_length.err

--- opening "vec_length.ma" ---
--- scope checking ---
--- type checking ---
type  Nat : Set
term  Nat.zero : < Nat.zero : Nat >
term  Nat.succ : ^(y0 : Nat) -> < Nat.succ y0 : Nat >
term  add : Nat -> Nat -> Nat
{ add Nat.zero y = y
; add (Nat.succ x) y = Nat.succ (add x y)
}
type  Vec : ++(A : Set) -> ^ Nat -> Set
term  Vec.vnil : .[A : Set] -> < Vec.vnil : Vec A Nat.zero >
term  Vec.vcons : .[A : Set] -> ^(y0 : A) -> .[n : Nat] -> ^(y2 : Vec A n) -> < Vec.vcons y0 n y2 : Vec A (Nat.succ n) >
term  length : .[A : Set] -> .[n : Nat] -> Vec A n -> Nat
error during typechecking:
length
/// clause 2
/// right hand side
/// checkExpr 5 |- succ n : Nat
/// checkForced fromList [(.(succ n),1),(A,0),(x,2),(n,3),(xs,4)] |- succ n : Nat
/// checkApp (^(y0 : Nat::()) -> < Nat.succ y0 : Nat >) eliminated by n
/// inferExpr' n
/// inferExpr: variable n : Nat may not occur
/// , because it is marked as erased