FUNCTION: enter - direct entry of classes in K₀ X

CALLING SEQUENCE:
enter(L,B_ind)

PARAMETERS:
L - a list (cf. below)
B_ind - the "Basis indicator", i.e. one of `@O`, `@S` or `@E`

SYNOPSIS:

• In order to make entering a coordinate vector as simple as possible coordinates are entered as a list, where entries corresponding to branches (e.g. of the canonical configuration) are in turn entered as lists. - Thus the first parameter of "enter", the coordinate vector, is entered as a list of t+2 items, where the first and last items are integers, while items 2,..,t+1 are lists of integers of length p₁-1,..,p_t-1 .

• The second parameter specifies the chosen basis to which the coordinate entries shall relate. - Please observe the ordering of each basis as indicated below, meaning that you have to replace each member of the chosen basis by its corresponding coordinate.

• As the result, the corresponding K₀ X class of type "quiver" is returned.

• for basis `@O`: (i.e. the indec. projective Lambda-modules)
  
  [O, [O(x₁),O(2x₁),..,O((p₁-1)x₁)], ... , [O(x_t),..,O((p_t-1)x_t)], O(c)]

• for basis `@E`: (i.e. the simple Lambda-modules) choose the same order as for the indec. projective Lambda-modules, indicated above, where each indec. projective module is replaced by the corresponding(!) simple module.

• for basis `@S`: (i.e. "simple sheaves")
  
  [O, [S_1,1,S_1,2,..,S_1,p1-1], ... , [S_t,1,..,S_t,pt-1], S ]
  
  where S_i,j denotes the cokernel of O(jx_i)--->O((j+1)x_i) .

EXAMPLES:

> with(tubular):

> read_in(2,3,6);

The following coordinates correspond to the indec. projective O(3 x3)

> L := [0,[0],[0,0],[0,0,1,0,0],0]; f := enter(L,`@O`); show(f);