[assembler] Accept U+2024 as a subscript dot.

It would be too confusing to accept ASCII "." as a subscript
character.

Author: youngman-g

assembler/src/asmlib/driver/tests.rs

@@ -668,12 +668,11 @@ fn default_assigned_index_register_easy_case() {
 
 #[test]
 fn test_bit_designator_evaluation_result() {
-    let program = assemble_source("100|SKM@sub_2@@sub_dot@@sub_1@ 0\n", Default::default())
-        .expect("program is valid");
+    let program = assemble_source("100|SKM₂․₁ 0\n", Default::default()).expect("program is valid");
     dbg!(&program);
     assert_eq!(program.chunks.len(), 1);
     assert_eq!(program.chunks[0].words.len(), 1);
-    // SKM is opcode 0o17.  bit designator 2.1 is 0o41.  The opcode
+    // SKM is opcode 0o17.  bit designator 2·1 is 0o41.  The opcode
     // gets shifted left 24 bit positions, the index valis is shiifted
     // left by 18 bit positions.
     assert_eq!(program.chunks[0].words[0], u36!(0o001_741_000_000));

assembler/src/asmlib/glyph.rs

@@ -843,7 +843,13 @@ const ALL_GLYPHS: &[Glyph] = &[
         // This is a centre dot, not a period.  We use a centre dot so
         // that it's not confused with a subscript dot.
         normal: Some('\u{00B7}'), // ·
-        subscript: None,
+
+        // Using an ASCII full stop / period (".") would be too
+        // confusing for the user, who (when preparing source code
+        // input) might expect this to be interpreted as the
+        // normal-script PERIOD.  So for subscript we instead use
+        // U+2024, "One Dot Leader".
+        subscript: Some('\u{2024}'), // "․" (not ASCII ".")
         superscript: None,
         ..GDEF
     },

assembler/src/asmlib/parser/tests.rs

@@ -235,6 +235,10 @@ fn test_subscript_literal_oct_decmode() {
         parse_successfully_with("₃₁@sub_dot@", literal(Script::Sub), set_decimal_mode),
         LiteralValue::from((span(0..15), Script::Sub, Unsigned36Bit::from(0o31_u32),))
     );
+    assert_eq!(
+        parse_successfully_with("₃₁․", literal(Script::Sub), set_decimal_mode),
+        LiteralValue::from((span(0..9), Script::Sub, Unsigned36Bit::from(0o31_u32),))
+    );
 }
 
 #[test]