anchor.diff:  .s-rule.do
   
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  -    **Do** create an Angular module for all distinct features in an application (e.g. `Heroes` feature).
  -
  -.s-rule.do
  -  :marked
  -    **Do** place the feature module in the same named folder as the feature area (.e.g `app/heroes`).
  -
  -.s-rule.do
  -  :marked
  -    **Do** name the feature module file reflecting the name of the feature area and folder (e.g. `app/heroes/heroes.module.ts`)
  -
  -.s-rule.do
  -  :marked
  -    **Do** name the feature module symbol reflecting the name of the feature area, folder, and file (e.g. `app/heroes/heroes.module.ts` defines `HeroesModule`)
  +    **Do** create an Angular module for all distinct features in an application; 
  +    for example, a `Heroes` feature.
  +
  +.s-rule.do
  +  :marked
  +    **Do** place the feature module in the same named folder as the feature area;
  +    for example, in `app/heroes`.
  +
  +.s-rule.do
  +  :marked
  +    **Do** name the feature module file reflecting the name of the feature area 
  +    and folder; for example, `app/heroes/heroes.module.ts`.
  +
  +.s-rule.do
  +  :marked
  +    **Do** name the feature module symbol reflecting the name of the feature 
  +    area, folder, and file; for example, `app/heroes/heroes.module.ts` defines `HeroesModule`.
   
   .s-why
     :marked
  
positive.code:   Some code examples display a file that has one or more similarly named companion files. 
    For example, `hero.component.ts` and `hero.component.html`.
  
    The guideline will use the shortcut `hero.component.ts|html|css|spec` to represent those various files. Using this shortcut makes this guide's file structures easier to read and more terse.
  
  .l-main-section
  a(id='toc')
  
  :marked
    ## Table of contents
  
      1. [Single responsibility](#single-responsibility)
      1. [Naming](#naming)
      1. [Coding conventions](#coding-conventions)
      1. [App structure and Angular modules](#application-structure-and-angular-modules)
      1. [Components](#components)
      1. [Directives](#directives)
      1. [Services](#services)
      1. [Data services](#data-services)
      1. [Lifecycle hooks](#lifecycle-hooks)
      1. [Appendix](#appendix)
  
  .l-main-section
  :marked
    ## Single responsibility
  
    Apply the 
    <a href="https://wikipedia.org/wiki/Single_responsibility_principle" target="_blank"><i>Single Responsibility Principle</i> (SRP)</a>
    to all components, services, and other symbols. 
    This helps make the app cleaner, easier to read and maintain, and more testable.
  
  a#rule-of-one
  :marked
    ### <a id="01-01"></a>_Rule of One_
    #### <a href="#01-01">Style 01-01</a>
  .s-rule.do
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negative.code: 
  .s-rule.do
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      **Do** be DRY (Don't Repeat Yourself).
  
  .s-rule.avoid
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