Toggle shift for single letter and add "keyboard layout" options.

t toggles case (a<->A) for whole word and TN toggles position N only.

S toggles shift (eg. a<->A and 4<->$) for whole word. It's a superset of t. Unfortunately there is no variant similar to TN for a single position.

I experimented with this (nearly identical to the TN code) that implements toggle shift of character at position N as WN, and it has proven useful.

diff --git a/src/rules.c b/src/rules.c
index 0016b97e1..308943779 100644
--- a/src/rules.c
+++ b/src/rules.c
@@ -1794,6 +1794,14 @@ char *rules_apply(char *word_in, char *rule, int split, char *last)
                        in[0] = 0;
                        break;
 
+               case 'W':
+                       {
+                               int pos;
+                               POSITION(pos)
+                               in[pos] = conv_shift[ARCH_INDEX(in[pos])];
+                       }
+                       break;
+
                case 'S':
                        CONV(conv_shift);
                        break;

I picked W because it was the first unused one (in qwerty order). @solardiz @jsteube is WN OK or would you prefer some other letter?