txt: ## mainline.md
  class X::Placeholder::Mainline
  
  Compilation error due to a placeholder in the mainline
  
  ```raku
  class X::Placeholder::Mainline is X::Placeholder::Block { }
  ```
  
  Thrown when a placeholder variable is used in the mainline, i.e. outside of any explicit block.
  
  For example
  
  ```raku
  $^x;
  
  ```
  
  dies with
  
  ｢text｣ without highlighting
  
  ```
  
  
  ```
  
  ===SORRY!===
  Cannot use placeholder parameter $^x outside of a sub or block
  
  
  ```
  
  
  ```
  
  Note that this error can also occur when you think something is a block, but it really is a [postcircumfix:<{ }>](/routine/{ }), for example
  
  ```raku
  my %h;
  say %h{ $^x };
  #     ^^^^^^^  not a block, so $^x is part of the mainline
  
  ```
  