Cond constructor
      
      Cond({ 
    
- @required dynamic ifExpr,
- @required dynamic thenExpr,
- @required dynamic elseExpr,
Creates $cond operator expression
Implementation
Cond({@required ifExpr, @required thenExpr, @required elseExpr})
    : super('cond', AEList([ifExpr, thenExpr, elseExpr]));