# QN Index

An Index object can be given additional quantum number (QN) block structure when it is constructed. This structure cannot be changed afterward.

In addition to the QN structure, a set of tags and an Arrow direction (In,Out) can be specified when constructing an Index.

## Synopsis

    // Construct an Index with QN information
    // 3 blocks, total size 4+8+4=16
    auto I = Index(QN({"Sz", 2}),4,
                   QN({"Sz", 0}),8,
                   QN({"Sz",-2}),4);

    // Provide Arrow direction and tags
    auto J = Index(QN({"T", 1}),3,
                   QN({"T",-1}),3,
                   In,"J,Link");

    // Make an Index with five blocks
    // and total size 4+8+10+8+4=34
    auto I = Index(QN(+2),4,
                   QN(+1),8,
                   QN( 0),10,
                   QN(-1),8,
                   QN(-2),4,
                   "I");

    Print(dim(I)); //prints: dim(I) = 16

    //Get number of blocks of I
    Print(nblock(I)); //prints: nblock(I) = 3

    //Get information about block 2
    Print(blocksize(I,2)); //prints: blocksize(I,2) = 8
    Print(qn(I,2));    //prints: QN({"Sz",0})

    //Get direction of I
    Print(dir(I)); //prints: dir(I) = Out

## Index properties ##

* `.dag()`

  `dag(Index I) -> Index`

  Change the arrow direction of the Index.

* `dir(Index I) -> Arrow`

  Return the `Arrow` direction of this Index.

* `nblock(Index I) -> int`

  Return the number of QN blocks of this Index.

* `qn(Index I, int j) -> QN`

  Return the QN of block `j`.

* `blocksize(Index I, int j) -> int`

  Return the block size of block `j`.