There's a bug: it gives error when the first aggregate
in the list is empty, see my updated version.
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diff --git a/src/algebra/aggcat.spad b/src/algebra/aggcat.spad
index 57b07e1f..b91bfbd4 100644
--- a/src/algebra/aggcat.spad
+++ b/src/algebra/aggcat.spad
@@ -1367,6 +1367,12 @@ UnaryRecursiveAggregate(S : Type) : Category == RecursiveAggregate S with
concat! : (%, S) -> %
++ concat!(u, x) destructively adds element x to the end of u.
++ Note: \spad{concat!(a, x) = concat!(a, [x])}.
+ concat! : List % -> %
+ ++ concat!(lu), where \spad{lu} is a list of aggregates
+ ++ \spad{[a, b, ..., c]}, returns a single aggregate consisting
+ ++ of the elements of \spad{a} followed by those of \spad{b}
+ ++ followed ... by the elements of \spad{c}. This function may
+ ++ destructively modify the aggregates in \spad{lu}.
cycleSplit! : % -> %
++ cycleSplit!(u) splits the aggregate by dropping off the cycle.
++ The value returned is the cycle entry, or empty() if none exists.
@@ -1561,6 +1567,17 @@ UnaryRecursiveAggregate(S : Type) : Category == RecursiveAggregate S with
setelt!(x, "rest", a) == setrest!(x, a)
concat(x : %, y : %) == concat!(copy x, y)
+ concat!(lu : List %) ==
+ while not empty? lu and empty? first lu repeat lu := rest lu
+ empty? lu => empty()
+ res := first lu
+ t := tail res
+ for u in rest lu repeat
+ not empty? u =>
+ setrest!(t, u)
+ t := tail t
+ res
+
-- The qxxx variants are for speed. If we do not care about
-- speed the regular ones will do.
