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.

com/ktext.cpp

@@ -1,6 +1,5 @@
 #include <iostream>
 #include "error.h"
-#include "check.h"
 #include "command.h"
 #include "log.h"
 #include "argv.h"
@@ -22,18 +21,20 @@ int main(int argc, char* argv[])
                  Target_registry::general_name, "'word'");
         args.opt("o", "Output basename; meaning and default defined by target.", "basename",
                  "", "'word'");
-        args.opt("k", "Klammerset symbol (resolved on the klammerset search path) or the pathname of a klammerset definition file; default is the Standard Klammer Set. With a value of \"none\", no klammerset is loaded.",
-                 "pathname", "", "'word'");
-        args.flag("d", "Display the output to the screen, rather than writing files.");
-        args.flag("m", "Show the Klammermachine state at the beginning of processing.");
-        args.flag("check", "Check every klammer application in the input and in the "
-                  "body of every defined klammer, report all problems found, and exit "
-                  "without producing output.");
+        args.flag("d", "Display the output to the screen, rather than writing a file.");
+        args.opt("klammersets", "Klammersets to load, as a list of symbols resolved on the "
+                 "klammerset search path.  Several combine: they are loaded in the order given "
+                 "and share one namespace.  The default is the Standard Klammer Set; "
+                 "\"none\" loads none.",
+                 "symbols", "", "'list'");
         args.opt("v", "'verbosity'", "degree", "0", "'verbosity'");
 
         if (show_usage(argc, argv)) {
+            // A bare command is a REQUEST for information, not a failure:
+            // usage goes to stdout (it is the result being asked for) and the
+            // exit status is 0, so "ktext && echo ok" reports what happened.
             args.usage(file_basename(argv[0]));
-            exit(1);
+            exit(0);
         }
         args.parse(argc, argv);
         if (verbose_level > 0) {
@@ -56,19 +57,6 @@ int main(int argc, char* argv[])
             parse_args(input_filenames, args.as_string("t"),
                        expand_tilde(args.as_string("o")), args.as_bool("d"));
 
-        // --check produces no output, so it needs neither a target nor -d.
-        // With no -t it checks every defined target, which is the more useful
-        // default here: "does this document hold together at all?"
-        bool check_only = args.as_bool("check");
-
-        if (*(output_filename.end() - 1) == '*'
-            && !display_only && !check_only) {
-            throw Argument_error(
-                "You must specify an output target or "
-                "display the results with the \"-d\" flag.",
-                Locator());
-        }            
-
         Machine M;
         M.m_state.open_frame("ktext");
         M.m_state.set("K_target", target);
@@ -89,20 +77,7 @@ int main(int argc, char* argv[])
             M.m_state.set("K_input_dir", fs::current_path().string());
         }
 
-        std::string klammerset_filename = args.as_string("k");
-        if (klammerset_filename != "none") {
-            if (klammerset_filename.empty()) {
-                klammerset_filename = M.m_state.value("KLAMMERTEXT_HOME") + "/sks/sks.k";
-            } else if (is_klammerset_symbol(klammerset_filename)) {
-                // A bare symbol resolves on the klammerset search path;
-                // the local stage is the input document's directory (the
-                // cwd when the input is a string).
-                klammerset_filename = resolve_klammerset_symbol(
-                    klammerset_filename, M.m_state.value("K_input_dir"), Locator()).string();
-            }
-            K::log(1, "Reading klammerset filename: "  + klammerset_filename);
-            M.read(fs::path(klammerset_filename));
-        }
+        load_klammersets(M, word_split(args.as_string("klammersets")));
 
         if (!input_text.empty()) {
             M.read(input_text + "\n");
@@ -111,30 +86,17 @@ int main(int argc, char* argv[])
             M.read(fs::path(p));
         }
 
-        if (args.as_bool("m")) {
-            std::cout << M << "\n";
-        }
-
-        // --check reports statically, before anything is applied: all problems
-        // at once, including ones in @cond branches that are not selected and
-        // in klammer bodies that this render would never reach.
-        if (check_only) {
-            int errors = report_diagnostics(check_machine(M, target), std::cout);
-            return errors > 0 ? 1 : 0;
-        }
-
         std::string result = trim(M.apply(target));
         if (display_only && !result.empty()) {
             std::cout << result << "\n";
         } else if (!result.empty()) {
-            msg() << "Output filename: " << output_filename << "\n";
             string_to_file(output_filename, result + "\n");
             (void)K::log(1, "Wrote file: " + output_filename);
         }
     }
     catch (Error& err) {
         err.print_message();
-        std::cout << "\n";
+        std::cerr << "\n";
         return 1;
     }
     return 0;