lexpr究竟是什么意思,以及r'/ F xx的含义是什么？什么是Application Expression？
from nltk.sem.logic import *
lexpr = Expression.fromstring
zero = lexpr(r'\F x.x')
one = lexpr(r'\F x.F(x)')
two = lexpr(r'\F x.F(F(x))')
three = lexpr(r'\F x.F(F(F(x)))')
four = lexpr(r'\F x.F(F(F(F(x))))')
succ = lexpr(r'\N F x.F(N(F,x))')
plus = lexpr(r'\M N F x.M(F,N(F,x))')
mult = lexpr(r'\M N F.M(N(F))')
pred = lexpr(r'\N F x.(N(\G H.H(G(F)))(\u.x)(\u.u))')
v1 = ApplicationExpression(succ, zero).simplify()