diff --git a/t/calc.t b/t/calc.t
index 0de002a..b8d1acd 100644
--- a/t/calc.t
+++ b/t/calc.t
@@ -6,18 +6,35 @@ use Test::Warnings ":all";
 
 BEGIN { use_ok('RevBank::Global'); }
 
-dies_ok sub { parse_amount("-1") };
-dies_ok sub { parse_amount("0-42") };
-dies_ok sub { parse_amount("999999") };
-
+# Invalid syntax
+is parse_amount(undef), undef;
+is parse_amount(""), undef;
 is parse_amount("0.123"), undef;
 is parse_amount("42.000"), undef;
 is parse_amount("a"), undef;
 is parse_amount("(1+1)"), undef;
+is parse_amount("+"), undef;
+is parse_amount("-"), undef;
+is parse_amount("++"), undef;
+is parse_amount("--"), undef;
+is parse_amount("+-"), undef;
+is parse_amount("-+"), undef;
+is parse_amount("1+"), undef;
+is parse_amount("1-"), undef;
+is parse_amount("1 1"), undef;
 
+# Out of range
+dies_ok sub { parse_amount("-1") };
+dies_ok sub { parse_amount("0-42") };
+dies_ok sub { parse_amount("999999") };
+
+# Bare number
 is parse_amount("42")->cents, 4200;
 is parse_amount("42.0")->cents, 4200;
 is parse_amount("42.00")->cents, 4200;
+
+
+# Arithmetic
 is parse_amount("1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1")->cents, 4200;
 
 is parse_amount("-42+42")->cents, 0;
diff --git a/t/fileio.t b/t/fileio.t
new file mode 100644
index 0000000..a227038
--- /dev/null
+++ b/t/fileio.t
@@ -0,0 +1,59 @@
+# These tests were written by ChatGPT. All four were actually correct the
+# first try.
+
+use Test::More;
+use File::Temp;
+
+use RevBank::FileIO;
+
+# ChatGPT didn't realise that ::FileIO doesn't export its functions
+use RevBank::Global;
+
+subtest "slurp" => sub {
+    my $tmp = File::Temp->new();
+    my $data = "foo\nbar\nbaz\n";
+    print $tmp $data;
+    close $tmp;
+    my @lines = slurp($tmp->filename);
+    is_deeply \@lines, ["foo\n", "bar\n", "baz\n"], "slurp works";
+};
+
+subtest "spurt" => sub {
+    my $tmp = File::Temp->new();
+    spurt($tmp->filename, "foo\nbar\nbaz\n");
+    open my $fh, "<", $tmp->filename;
+    local $/;
+    my $contents = <$fh>;
+    close $fh;
+    is $contents, "foo\nbar\nbaz\n", "spurt works";
+};
+
+subtest "append" => sub {
+    my $tmp = File::Temp->new();
+    spurt($tmp->filename, "foo\n");
+    append($tmp->filename, "bar\n", "baz\n");
+    open my $fh, "<", $tmp->filename;
+    local $/;
+    my $contents = <$fh>;
+    close $fh;
+    is $contents, "foo\nbar\nbaz\n", "append works";
+};
+
+subtest "rewrite" => sub {
+    my $tmp = File::Temp->new();
+    spurt($tmp->filename, "foo\nbar\nbaz\n");
+    rewrite($tmp->filename, sub {
+        my ($line) = @_;
+        if ($line =~ /^bar/) {
+            return "quux\n";
+        }
+        return $line;
+    });
+    open my $fh, "<", $tmp->filename;
+    local $/;
+    my $contents = <$fh>;
+    close $fh;
+    is $contents, "foo\nquux\nbaz\n", "rewrite works";
+};
+
+done_testing();