In incidence structures, particularly simple ones, blocks are basically sets. For this reason, the elementary set operations such as join, meet and subset have been made to work on blocks. However, blocks are not sets, and so the functions {tt Set} and Support below have been provided to convert a block to a set of points for other uses.

p in B : IncPt, IncBlk -> BoolElt

True if point p lies in block B, otherwise false.

True if point p does not lie in block B, otherwise false.

Given a subset S of the point set of the incidence structure D and a block B of D, return true if the subset S of points lies in B, otherwise false.

Given a subset S of the point set of the incidence structure D and a block B of D, return true if the subset S of points does not lie in B, otherwise false.

The number of blocks that contains the point p.

# B : IncBlk -> RngIntElt

The number of points contained in the block B.

The set of points contained in the block B.

The set of underlying points contained in the block B (i.e. the elements of the set have their "real" types; they are no longer from the category IncPt).

IsBlock(S, D) : SetEnum, Inc -> BoolElt, IncBlk

True iff the set (or block) S represents a block of the incidence structure D. If true, also returns one such block.

Block(p, q) : IncPt, IncPt -> IncBlk

A block containing the points p and q (if one exists). In linear spaces, such a block exists and is unique (assuming p and q are different).

True iff there exists a parallel class containing the blocks B and C; if true, also returns one such class.

The connection number c(p, B); i.e. the number of blocks joining p to B.

> D, P, B := Design< 2, 7 | {3, 5, 6, 7}, {2, 4, 5, 6}, {1, 4, 6, 7},
>   {2, 3, 4, 7}, {1, 2, 5, 7}, {1, 2, 3, 6}, {1, 3, 4, 5} >;
> print D: Maximal;
2-(7, 4, 2) Design with 7 blocks
Points: {@ 1, 2, 3, 4, 5, 6, 7 @}
Blocks:
    {3, 5, 6, 7},
    {2, 4, 5, 6},
    {1, 4, 6, 7},
    {2, 3, 4, 7},
    {1, 2, 5, 7},
    {1, 2, 3, 6},
    {1, 3, 4, 5}
> print P.1 in B.1; 
false
> print P.1 in B.3;
true
> print {P| 1, 2} in B.5;
true
> print Block(P.1, P.2);
{1, 2, 5, 7}
> b := B.4;              
> print b;
{2, 3, 4, 7}
> print b meet {2, 8};
{ 2 }
> S := Set(b);
> print S, Universe(S);
{ 2, 3, 4, 7 }
The point-set of 2-(7, 4, 2) Design with 7 blocks
> Supp := Support(b);
> print Supp, Universe(Supp);
{ 2, 3, 4, 7 }
Integer Ring