Singular

LIB "grobcov.lib";
if(defined(R)){kill R;}
ring R=(0,x,y,z,t),(x1,y1),lp;
ideal a1=x;
ideal a2=x,y;
ideal a3=x,y,z;
ideal a4=x,y,z,t;
ideal b1=y;
ideal b2=y,z;
ideal b3=y,z,t;
ideal b4=1;
list L1=a1,a2,a3,a4;
list L2=b1,b2,b3,b4;
L1;
==> [1]:
==>    _[1]=(x)
==> [2]:
==>    _[1]=(x)
==>    _[2]=(y)
==> [3]:
==>    _[1]=(x)
==>    _[2]=(y)
==>    _[3]=(z)
==> [4]:
==>    _[1]=(x)
==>    _[2]=(y)
==>    _[3]=(z)
==>    _[4]=(t)
L2;
==> [1]:
==>    _[1]=(y)
==> [2]:
==>    _[1]=(y)
==>    _[2]=(z)
==> [3]:
==>    _[1]=(y)
==>    _[2]=(z)
==>    _[3]=(t)
==> [4]:
==>    _[1]=1
def LL=DifConsLCSets(L1,L2);
LL;
==> [1]:
==>    [1]:
==>       _[1]=(x)
==>    [2]:
==>       _[1]=(y)
==>       _[2]=(x)
==> [2]:
==>    [1]:
==>       _[1]=(z)
==>       _[2]=(y)
==>       _[3]=(x)
==>    [2]:
==>       _[1]=(t)
==>       _[2]=(z)
==>       _[3]=(y)
==>       _[4]=(x)
def LLL=ConsLevels(LL);
LLL;
==> [1]:
==>    _[1]=(x)
==> [2]:
==>    _[1]=(y)
==>    _[2]=(x)
==> [3]:
==>    _[1]=(z)
==>    _[2]=(y)
==>    _[3]=(x)
==> [4]:
==>    _[1]=(t)
==>    _[2]=(z)
==>    _[3]=(y)
==>    _[4]=(x)
==> [5]:
==>    _[1]=1
def LLLL=Levels(LLL);
LLLL;
==> [1]:
==>    [1]:
==>       1
==>    [2]:
==>       [1]:
==>          _[1]=(x)
==>       [2]:
==>          _[1]=(y)
==>          _[2]=(x)
==> [2]:
==>    [1]:
==>       3
==>    [2]:
==>       [1]:
==>          _[1]=(z)
==>          _[2]=(y)
==>          _[3]=(x)
==>       [2]:
==>          _[1]=(t)
==>          _[2]=(z)
==>          _[3]=(y)
==>          _[4]=(x)