Update leading-separator.md (#3192)

proposals/csharp-7.2/leading-separator.md

@@ -1,17 +1,32 @@
 # Allow digit separator after 0b or 0x
 
-In C# 7.2, we extend the set of places that digit separators (the underscore character) can appear in integral literals. [Beginning in C# 7.0, separators are permitted between the digits of a literal](../csharp-7.0/digit-separators.md). Now, in C# 7.2, we also permit digit separators before the first significant digit of a binary or hexadecimal literal, after the prefix.
+This proposal specifies the changes required to the [C# 7.1 (draft) Language specification](XXX) to support leading underscores in binary and hex literals. It builds on top of the changes added in 7.0 when binary literals were introduced.
 
-```csharp
-    123      // permitted in C# 1.0 and later
-    1_2_3    // permitted in C# 7.0 and later
-    0x1_2_3  // permitted in C# 7.0 and later
-    0b101    // binary literals added in C# 7.0
-    0b1_0_1  // permitted in C# 7.0 and later
+## Changes to [Lexical structure](../../spec/lexical-structure.md)
 
-    // in C# 7.2, _ is permitted after the `0x` or `0b`
-    0x_1_2   // permitted in C# 7.2 and later
-    0b_1_0_1 // permitted in C# 7.2 and later
+### Literals
+
+#### Integer literals
+
+> The grammar for [integer literals](../../spec/lexical-structure.md#Integer-literals) is modified to allow one or more `_` separators before the first digit of a hex or binary literal.
+
+```antlr
+hex_digits
+    : '_'? hex_digit
+    | '_'? hex_digit hex_digits_and_underscores? hex_digit
+    ;
+
+binary_digits
+    : '_'? binary_digit
+    | '_'? binary_digit binary_digits_and_underscores? binary_digit
+    ;
 ```
 
-We do not permit a decimal integer literal to have a leading underscore. A token such as `_123` is an identifier.
+> Make the following changes to the examples:
+
+\[Example:
+```csharp
+0x_abc               // hex, int
+0B__111              // binary, int
+```
+end example\]