An output policy for the three commands, and @cond as a true special form

A snapshot of the development tree.  The substantial changes since the last one:

COMMAND OUTPUT POLICY.  The three commands display text in exactly three cases,
and each owns a stream: LOGGING under "-v" greater than 0 and an ERROR before
termination go to STDERR; OUTPUT THE USER ASKED FOR goes to STDOUT.  For ktext
that output is a document, so "ktext doc.kt -d | ..." is now safe -- logging
used to share the stream and land inside the document.  A bare command prints
its usage and succeeds rather than failing.  Colour is emitted only to a
terminal, per stream, and NO_COLOR is honoured.

"-v 1" reports every decision whose outcome you could not have read off your own
input: the klammerset that was loaded and from which file, a font's directory, a
":files" name's file, how "-o" was expanded.  Higher levels are the trace.

The commands no longer warn and continue: an anomaly is an error, described with
its location.  Two exceptions remain, each for a stated reason -- a condition
that is expected and temporary by design, and a judgment that is a heuristic
rather than exact.

@cond IS NOW A TRUE SPECIAL FORM, resolved at APPLICATION time rather than when
the file is read.  Two consequences for a writer:

  * a state variable reaches the predicate.  "@@@state Flag :value true @@@
    @cond *Flag* | T | F @" renders "T"; it used to see the literal "*Flag*" and
    silently take the false branch.  The document now behaves like a klammer
    body, whose arguments are bound before its conditionals are decided.
  * nothing in a discarded branch happens -- it is not read, not evaluated, not
    expanded.  An @eval in the branch not taken used to run anyway.

Its predicate relation is total and strict: true, True, 1; false, False, 0, and
empty; anything else is an error at the @cond rather than silently false.

@eval REACHING OUTSIDE.  ":shell" and ":haskell" now keep the command's standard
error out of the document (it appears under "-v 1") and treat a nonzero exit as
an error naming what the command reported.  A command that exits nonzero on
purpose -- "grep" finding no match -- says so with "|| true".

KLAMMER SETS.  Several combine: "--klammersets a b c" loads all three in the
order given, sharing one namespace, with the definition modes deciding
collisions.  "none" means none and may not be combined with other symbols.  A
klammerset with symbol X is declared in a file X/X.k, which is what lets two
sets require the same third set without loading it twice.

TESTS.  Four new suites: the kdiag command's interface, the @eval primitive's
contract with the outside world, and verbosity at both tiers.  Three suites
that could not run on macOS at all now do.

Assembled from dev commit 6c8ee6c22fca.

mac/command.cpp

@@ -3,6 +3,7 @@
 #include "argv.h"
 #include "file.h"
 #include "log.h"
+#include "machine.h"
 
 using namespace std::string_literals;
 
@@ -109,3 +110,48 @@ parse_args(
     }
     return {output_target, output_dir, output_basename, output_filename, write_files, display_only};
 }
+
+
+void load_klammersets(Machine& machine, const strings_t& symbols)
+{
+    // "none" is the absence of a klammerset, not the name of one, so it says
+    // nothing about what should be loaded ALONGSIDE it: written first it would
+    // silently discard the rest of the list, written last it would be looked up
+    // as a symbol and reported missing.  Both readings are wrong, so neither is
+    // guessed at -- "none" is accepted only alone.
+    if (is_in("none"s, symbols) && symbols.size() > 1) {
+        throw Klammerset_error(
+            "The klammerset \"none\" cannot be combined with other klammersets: "
+            "it means that none is loaded. Give \"none\" alone, or name only the "
+            "klammersets to load.",
+            Locator());
+    }
+    if (symbols.empty()) {
+        // Which klammerset was loaded, and from where, is a DERIVED value: the
+        // user wrote no --klammersets at all and got the SKS from
+        // $KLAMMERTEXT_HOME.  Only the symbol branch below reported its
+        // resolution, so the commonest case -- the default -- said nothing.
+        std::string sks = machine.m_state.value("KLAMMERTEXT_HOME") + "/sks/sks.k";
+        (void)K::log(1, "Klammerset (default): " + sks);
+        machine.read(fs::path(sks));
+    } else if (symbols[0] == "none") {
+        (void)K::log(1, "Klammersets: none");
+    } else {
+        // The whitespace-delimited words in symbols must be klammerset symbols:
+        for (const auto& symbol : symbols) {
+            // Same guard as ":requires" (Machine::load_klammerset_files): a set
+            // named twice on the command line is loaded once.
+            if (machine.m_klammersets.has(symbol)) {
+                (void)K::log(1, "Klammerset \"" + symbol + "\": already loaded, skipped");
+                continue;
+            }
+            std::string klammerset_filename = resolve_klammerset_symbol(
+                symbol, machine.m_state.value("K_input_dir"), Locator()).string();
+            // Symbol -> file is the search path's answer, and the search
+            // path has three stages with shadowing: the file it landed on is
+            // exactly what a user cannot read off "--klammersets x".
+            (void)K::log(1, "Klammerset \"" + symbol + "\": " + klammerset_filename);
+            machine.read(fs::path(klammerset_filename));
+        }
+    }
+}