Prevent names like _99 in the spec

1 files changed, 1 insertions, 1 deletions

diff --git a/spec/token.md b/spec/token.md
index 93dd1dfa..91608c47 100644
--- a/spec/token.md
+++ b/spec/token.md
@@ -10,7 +10,7 @@ A BQN *character literal* consists of a single character between single quotes,
 
 A comment consists of the hash character `#` and any following text until (not including) the next newline character. The initial `#` must not be part of a string literal started earlier. Comments are ignored entirely and do not form tokens.
 
-Identifiers and numeric literals share the same token formation rule. These tokens are formed from the *numeric characters* `¯∞π.0123456789` and *alphabetic characters* `_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ` and the oddball `𝕣`. Any sequence of these characters adjacent to each other forms a single token, which is a *numeric literal* if it begins with a numeric character and an *identifier* if it begins with an alphabetic character. Numeric literals are also subject to [numeric literal rules](literal.md), which specify which numeric literals are valid and which numbers they represent. If the token contains `𝕣` it must be either `𝕣`, `_𝕣`, or `_𝕣_` and is considered a special name (see below). As the value taken by this identifier can only be a modifier, the uppercase character `ℝ` is not allowed.
+Identifiers and numeric literals share the same token formation rule. These tokens are formed from the *numeric characters* `¯∞π.0123456789` and *alphabetic characters* `_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ` and the oddball `𝕣`. Any sequence of these characters adjacent to each other forms a single token, which is a *numeric literal* if it begins with a numeric character and an *identifier* if it begins with an alphabetic character. If a token begins with an underscore then its first non-underscore character must be alphabetic: for example, `_99` is not a valid token. Numeric literals are also subject to [numeric literal rules](literal.md), which specify which numeric literals are valid and which numbers they represent. If the token contains `𝕣` it must be either `𝕣`, `_𝕣`, or `_𝕣_` and is considered a special name (see below). As the value taken by this identifier can only be a modifier, the uppercase character `ℝ` is not allowed.
 
 Following this step, the whitespace characters space and tab are ignored, and do not form tokens. Only these whitespace characters, and the newline character, which does form a token, are allowed.