docs: update syntax

docs/syntax.md

@@ -8,10 +8,10 @@ English | [简体中文](./syntax.zh-Hans.md)
 
 A *Chemical Structure* starts with a *Group*. A *Group* can:
 
-- Include uppercase and lowercase characters, lowercase characters following uppercase characters will be merged into one font box (to make element notation more natural)
+- Include uppercase and lowercase characters, lowercase characters following uppercase characters will be merged into one text box (to make element notation more natural)
 - Include digits, digits will be rendered as subscript
 - Be an isolated *Wildcard `?`*, and will not be rendered (to express a substituent)
-- Be an isolated *Collapsed Carbon Atom `.`*, and will not be rendered, connected bonds will be rendered without gaps
+- Start with a *Collapsing Flag `.`* to make the text box collapse. Connected *Bonds* will be rendered without gaps when there's nothing after `.` (like bond-line formulas)
 
 A *Group* can include *In-group Typesetting*:
 
@@ -54,17 +54,26 @@ Rules of expressing a *Bond*:
 *Bond Directions* have a similar expressing method to writing chemical structure:
 
 - Using `-`, `|`, `/` and `\` to represent a *Bond* of 0°, 90°, 60° and 270° respectively
+- Using `@` followed by a number to represent a *Bond* of specific degree. The degree can be positive or negative, integer or decima. At end it will be converted to a value between 0° and 360°
 - *Bond Directions* can be `+`, see [Overlapping](#bond-overlapping) for details
 - If *Bond Type* appears before the *Chemical Structure* it connected, the direction will follow the rules stated above; if *Bond Type* appears after the *Chemical Structure* it connected, the direction will be the opposite (i.e. adding a deflection of 180°)
 - If *Bond Type* appears after the *Chemical Structure* it connected, the *Bond* can only appear in a *Bonds Set*
-- *Bond Direction* must not be duplicated (bonds in), and must be unique from the direction of the *Bond* that it is connected from the *Group* it belongs to
+- *Bond Direction* (of *Bonds out*) must not be duplicated, and must be unique from the direction of the *Bond* that it is connected from the *Group* it belongs to (i.e. *Bond in*)
 
 *Bond Modifier*:
 
-- *Bond Modifier `*`* sets the length (`length` *Attribute*) of the *Bond* to 0
+- *Bond Modifier `*`* sets the length (*Attribute `length`*) of the *Bond* to 0
 - *Bond Modifier `!`* can make all the following *Bond Directions* add a deflection of 180° (to express opposite *Bonds* outsides the *Bonds Set*)
 - *Bond Modifier `~`* acts on all *Bond Directions* of `/` and `\` in the following *Bond Directions*. It can make their angle with x-axis change from 60° to 30°
 
+Shorthand rules for *Bond Modifier `~`*:
+- When `~` is not followed by *Bond Direction*, the direction will be automatically infered based on the direction of *Bond in*
+- 若没有入键，方向推导为 `~/`；若同时存在「键修饰符 `!`」，方向推导为 `!~\`
+- When there is no *Bond in*, the direction will be inferred to `~/`; when *Bond Modifier `!`* also exists, the direction will be inferred to `!~\`
+- When there is a *Bond in*, `~/` and `~\` will be inferred to each other; when *Bond Modifier `!`* also exists, `!~\` and `!~/` will be inferred to each other 
+- When there is a *Bond in* but its direction is not among the above four directions, an inferring error will occur
+- This shorthand rule is designed for expressing carbon chains in bond-line formulas
+
 Shorthand rules for *Bond Directions*:
 
 - If *Bond Count* exists, omitting *Bond Direction* is allowed, in this case, *Bond Direction* will be 0° by default
@@ -79,11 +88,16 @@ Overlapping more than one *Bond Direction* is allowed to express connecting to t
 - If a *Bond Direction* and the first *Bond Direction* are symmetrical about the x-axis/y-axis, all the *Bonds* in the *Chemical Structure* connected to the former will be flipped by the x-axis/y-axis (however, the texts in the *Groups* in it will not be rotated)
 - All *Bonds* in *Chemical Structure* connected by other *Bond Direction* will be rotated by the same angle as the angle between their *Bond Direction* and the first *Bond Direction*
 
+Overlapping `@` (experimental)
+- `@!` means an extra direction with the opposite direction
+- `@-` means an extra direction with the x-flipped direction
+- `@|` means an extra direction with the y-flipped direction
+
 ## Referencing
 
 *Referencing* can be used to separate complicated *Chemical Structures* into parts to express
 
-- A *Chemical Structure* can be named by using *Attribute* `ref`, a named *Chemical Structure* is referenceable
+- A *Chemical Structure* can be named by using *Attribute `ref`*, a named *Chemical Structure* is referenceable
 - A sign `&` followed by a referenced name is a *Referencing*, as well as a *Group*, it can be followed by *Bonds*
 - Using `;` to separate different *Chemical Structures*. (Dangling `;` is allowed) Ultimately, these *Chemical Structures* must be fully connected by *Referencing*
 - A *Referencing* cannot be connected to itself by a *Bond* (i.e., self-loops are not allowed)
@@ -100,13 +114,14 @@ Overlapping more than one *Bond Direction* is allowed to express connecting to t
 
 *Bond* Attributes:
 
-| Attribute Name | Alias     | Type              | Usage
-| :------------- | :-------- | :---------------- | :----
-| `color`        | `C`       | String            | To specify the bond's color
-| `highEnergy`   | `HE`, `~` | Boolean           | To describe a high-energy bond
-| `from`         | `<`       | Boolean / Integer | Coordinate bond came from along the bond, an integer can specify the number of arrows
-| `to`           | `>`       | Boolean / Integer | Coordinate bond came to along the bond, an integer can specify the number of arrows
-| `length`       | `L`       | Float             | To specify the bond's length (multiple of unit length, 1 by default)
+| Attribute Name | Alias     | Type               | Usage
+| :------------- | :-------- | :----------------- | :----
+| `color`        | `C`       | String             | To specify the bond's color
+| `highEnergy`   | `HE`, `~` | Boolean            | To describe a high-energy bond
+| `from`         | `<`       | Boolean / Integer  | Coordinate bond came from along the bond, an integer can specify the number of arrows
+| `to`           | `>`       | Boolean / Integer  | Coordinate bond came to along the bond, an integer can specify the number of arrows
+| `length`       | `L`       | Float              | To specify the bond's length (multiple of unit length, 1 by default)
+| `side`         | `S`       | Constant `L` / `R` | To specify the bond's inside. `L` means left and `R` means right. This determines which side the second bond of a double-bond appears 
 
 ## Form
 

docs/syntax.zh-Hans.md

@@ -10,7 +10,7 @@
 - 包含大小写字母，大写字母后的小写字母会与之合并在一个字框内（使元素符号更自然）
 - 包含数字，数字会被渲染为下标形式
 - 是一个单独的「通配符 `?`」，不渲染（用于表示取代基）
-- 是一个单独的「塌缩碳原子 `.`」，不渲染，与之相连的键不会留白（类似键线式）
+- 以「塌缩标志 `.`」开头，这会使字框塌缩。`.` 后无内容时，相连的「键」会无缝衔接（类似键线式）
 
 「原子团」可以包含「原子团内排版」：
 - 用 `^`/`_`/`` ` `` 表示上标/下标/正常显示其后的一个字符
@@ -51,18 +51,18 @@
 - 「键方向」可以为 `+`，见[重迭书写规则](#键的重迭书写)
 - 当「键类型」写在连接的「化学结构」前时，方向即如上条所说；写在其后时，则反向（加上 180°）
 - 「键类型」写在连接的「化学结构」后时，只能出现在「键组」内
-- 「键方向」不得重复（出键）；也不得与连接其所属的「原子团」的「键」的方向（入键）相反
+- （「出键」的）「键方向」不得重复；也不得与连接其所属的「原子团」的「键」（即「入键」）的方向相反
 
 「键修饰符」：
-- 「键修饰符 `*`」将「键」的长度（`length` 「属性」)）设置为 0
+- 「键修饰符 `*`」将「键」的长度（「属性 `length`」)）设置为 0
 - 「键修饰符 `!`」使其后的「键方向」都增加 180°（这样可以在「键组」外书写反方向的键）
 - 「键修饰符 `~`」使其后的「键方向」中，为 `/` 或 `\` 的，与 x 轴夹角从 60° 变为 30°
 
 「键修饰符 `~`」的简写规则：
-- 当 `~` 后不跟随「键方向」时，会根据其所属「原子团」的入键方向自动推导方向
-- 若没有入键，方向推导为 `~/`；若同时存在「键修饰符 `!`」，方向推导为 `!~\`
-- 若有入键，则按照 `~/` 对应 `~\` 推导；若同时存在「键修饰符 `!`」，则按照 `!~\` 对应 `!~/` 推导
-- 若有入键且其方向不在上述 4 个方向中，则产生推导错误
+- 当 `~` 后不跟随「键方向」时，会根据「入键」方向自动推导方向
+- 若没有「入键」，方向推导为 `~/`；若同时存在「键修饰符 `!`」，方向推导为 `!~\`
+- 若有「入键」，则按照 `~/` 对应 `~\` 推导；若同时存在「键修饰符 `!`」，则按照 `!~\` 对应 `!~/` 推导
+- 若有「入键」且其方向不在上述 4 个方向中，则产生推导错误
 - 这一简写规则是为键线式中书写碳链设计的
 
 「键方向」的简写规则：
@@ -86,7 +86,7 @@
 
 「引用」可以用于将复杂的「化学结构」拆分开来表示
 
-- 用 `ref` 「属性」给某个「化学结构」命名，从而可被引用
+- 用「属性 `ref`」给某个「化学结构」命名，从而可被引用
 - 把引用名称写在 `&` 符号后面，就构成一个「引用」，其可以像「原子团」一样跟随「键」
 - 用 `;` 隔开不同的「化学结构」。（允许悬挂的 `;`）最终，通过「引用」的连接，这些「化学结构」必须完全连通
 - 「引用」不能通过「键」连接到它本身（即不允许出现自环）