Introduce the concept of nodes that are semantically transparent:

they are skipped when looking for previous or following high-level
macros.  Examples include roff(7) .ft, .ll, and .ta, mdoc(7) .Sm
and .Tg, and man(7) .DT and .PD.  Use this concept for a variety
of improved decisions in various validators and formatters.

While here,
* remove a few const qualifiers on struct arguments that caused trouble;
* get rid of some more Yoda notation in the vicinity;
* and apply some other stylistic improvements in the vicinity.

I found this class of issues while considering .Tg patches from kn@.

5 files changed, 191 insertions, 3 deletions

diff --git a/regress/mdoc/blank/Makefile b/regress/mdoc/blank/Makefile
index 7dfd0287..8b61f902 100644
--- a/regress/mdoc/blank/Makefile
+++ b/regress/mdoc/blank/Makefile
@@ -1,7 +1,7 @@
-# $OpenBSD: Makefile,v 1.5 2014/07/06 19:08:57 schwarze Exp $
+# $OpenBSD: Makefile,v 1.8 2020/02/27 01:25:59 schwarze Exp $
 
-REGRESS_TARGETS = line comment list
-LINT_TARGETS	= line comment list
+REGRESS_TARGETS = line comment list transp
+LINT_TARGETS	= line comment list transp
 
 SKIP_TMAN = list
 SKIP_MARKDOWN ?= line
diff --git a/regress/mdoc/blank/transp.in b/regress/mdoc/blank/transp.in
new file mode 100644
index 00000000..0e58bf2e
--- /dev/null
+++ b/regress/mdoc/blank/transp.in
@@ -0,0 +1,77 @@
+.\" $OpenBSD: transp.in,v 1.1 2020/02/27 01:25:59 schwarze Exp $
+.Dd $Mdocdate$
+.Dt BLANK-TRANSP 1
+.Os
+.Sh NAME
+.Nm blank-transp
+.Nd transparent nodes between line breaks
+.Sh DESCRIPTION
+Double br:
+.br
+.Tg brbr
+.br
+br Pp:
+.br
+.Tg brPp
+.Pp
+Pp br:
+.Pp
+.Tg Ppbr
+.br
+Double Pp:
+.Pp
+.Tg PpPp
+.Pp
+br sp:
+.br
+.Tg brsp
+.sp
+sp br:
+.sp
+.Tg spbr
+.br
+Pp sp:
+.Pp
+.Tg Ppsp
+.sp
+Pp sp 2v:
+.Pp
+.Tg Ppsp2v
+.sp 2v
+sp Pp:
+.sp
+.Tg spPp
+.Pp
+Double sp:
+.sp
+.Tg spsp
+.sp
+br blank:
+.br
+.Tg brbl
+
+blank br:
+
+.Tg blbr
+.br
+Pp blank:
+.Pp
+.Tg Ppbl
+
+blank Pp:
+
+.Tg blPp
+.Pp
+sp blank:
+.sp
+.Tg spbl
+
+blank sp:
+
+.Tg blsp
+.sp
+Double blank:
+
+.Tg blbl
+
+end of file
diff --git a/regress/mdoc/blank/transp.out_ascii b/regress/mdoc/blank/transp.out_ascii
new file mode 100644
index 00000000..f8bdd6b8
--- /dev/null
+++ b/regress/mdoc/blank/transp.out_ascii
@@ -0,0 +1,48 @@
+BLANK-TRANSP(1)             General Commands Manual            BLANK-TRANSP(1)
+
+NNAAMMEE
+     bbllaannkk--ttrraannsspp - transparent nodes between line breaks
+
+DDEESSCCRRIIPPTTIIOONN
+     Double br:
+     br Pp:
+
+     Pp br:
+
+     Double Pp:
+
+     br sp:
+
+     sp br:
+
+     Pp sp:
+
+     Pp sp 2v:
+
+     sp Pp:
+
+
+     Double sp:
+
+
+     br blank:
+
+     blank br:
+
+     Pp blank:
+
+     blank Pp:
+
+
+     sp blank:
+
+
+     blank sp:
+
+
+     Double blank:
+
+
+     end of file
+
+OpenBSD                        February 27, 2020                       OpenBSD
diff --git a/regress/mdoc/blank/transp.out_lint b/regress/mdoc/blank/transp.out_lint
new file mode 100644
index 00000000..533ea600
--- /dev/null
+++ b/regress/mdoc/blank/transp.out_lint
@@ -0,0 +1,19 @@
+mandoc: transp.in:52:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:54:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:60:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:62:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:68:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:70:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:74:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:76:1: WARNING: blank line in fill mode, using .sp
+mandoc: transp.in:12:2: WARNING: skipping paragraph macro: br after br
+mandoc: transp.in:14:2: WARNING: skipping paragraph macro: br before Pp
+mandoc: transp.in:20:2: WARNING: skipping paragraph macro: br after Pp
+mandoc: transp.in:22:2: WARNING: skipping paragraph macro: Pp before Pp
+mandoc: transp.in:26:2: WARNING: skipping paragraph macro: br before sp
+mandoc: transp.in:32:2: WARNING: skipping paragraph macro: br after sp
+mandoc: transp.in:36:2: WARNING: skipping paragraph macro: sp after Pp
+mandoc: transp.in:40:2: WARNING: skipping paragraph macro: sp after Pp
+mandoc: transp.in:50:2: WARNING: skipping paragraph macro: br before sp
+mandoc: transp.in:56:2: WARNING: skipping paragraph macro: br after sp
+mandoc: transp.in:60:1: WARNING: skipping paragraph macro: sp after Pp
diff --git a/regress/mdoc/blank/transp.out_markdown b/regress/mdoc/blank/transp.out_markdown
new file mode 100644
index 00000000..9c8ac825
--- /dev/null
+++ b/regress/mdoc/blank/transp.out_markdown
@@ -0,0 +1,44 @@
+BLANK-TRANSP(1) - General Commands Manual
+
+# NAME
+
+**blank-transp** - transparent nodes between line breaks
+
+# DESCRIPTION
+
+Double br:  
+br Pp:
+
+Pp br:
+
+Double Pp:
+
+br sp:
+
+sp br:
+
+Pp sp:
+
+Pp sp 2v:
+
+sp Pp:
+
+Double sp:
+
+br blank:
+
+blank br:
+
+Pp blank:
+
+blank Pp:
+
+sp blank:
+
+blank sp:
+
+Double blank:
+
+end of file
+
+OpenBSD - February 27, 2020