Typescript <；的字符是什么意思打字稿？

问题:

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• 字符在处的含义是什么这不是TypeScript。它的意思似乎是“是的一个子类型”
  例如，

  interface Literal如果S是T的子类型，子类型关系通常写为S
                    This is not TypeScript. It looks to mean "is a subtype of".
  
  For example, 
  interface Literal <: Expression
   means that a 
  Literal
   is a type of 
  Expression. Similarly, 
  interface Expression <: Node
   means that an 
  Expression
   is a type of 
  Node
  . (In the abstract syntax tree, every element in the tree is some type of Node)
  
  When there are multiple identifiers on the right, eg:
  
  interface FunctionExpression <: Function, Expression