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+#!/usr/bin/env perl
+# Perl weekly challenge 257
+# Task 2:  Reduced Row Echelon. More compact, eliminate Perl loop in
+#                               PDL solution
+#
+# See https://wlmb.github.io/2024/02/20/PWC257/#task-2-reduced-row-echelon
+use v5.36;
+use PDL;
+use PDL::NiceSlice;
+die <<~"FIN" unless @ARGV;
+    Usage: $0 M1 [M2...]
+    to check if the matrices Mn is a reduced row echelon matrix.
+    The matrices are strings of the form
+    "[[M11 M12..][M21 M22...]...]" where each Mij is a number,
+    the element in the i-th row and j-th column.
+    FIN
+for(@ARGV){
+    my $matrix=pdl($_);
+    my $extended_matrix=pdl(0)->glue(0, $matrix, identity($matrix->dim(1)));
+    my ($freq,$vals)=$extended_matrix->rle; # run length encode
+    $freq=$freq((0));                       # number of leading zeroes
+    my $result=all($freq==$freq->qsort)     # check non-decreasing
+                                            # check leading 1 and 0
+	                                    # elsewhere in column
+	&& all($extended_matrix->index1d($freq)==identity($freq->dim(0)));
+    say "$matrix -> $result"
+}