Function: mseisenstein
Section: modular_symbols
C-Name: mseisenstein
Prototype: G
Help: mseisenstein(M): M being a full modular symbol space, as given by msinit,
 return its Eisenstein subspace.
Doc:
 $M$ being a full modular symbol space, as given by \kbd{msinit},
 return its Eisenstein subspace.
 A subspace is given by a structure allowing quick projection and
 restriction of linear operators; its first component is
 a matrix with integer coefficients whose columns form a $\Q$-basis of
 the subspace.
 This is the same basis as given by the second component of
 \kbd{mscuspidal}$(M, 1)$.
 \bprog
 ? M = msinit(2,8, 1); \\ M_8(Gamma_0(2))^+
 ? E = mseisenstein(M);
 ? E[1]  \\ 2-dimensional
 %3 =
 [0 -10]

 [0 -15]

 [0  -3]

 [1   0]

 ? E == mscuspidal(M,1)[2]
 %4 = 1
 @eprog
