Description: Theorem 9.20 of [Laboreo] p. 43, translated line by line using the
usual translation of natural deduction (ND) in the
Metamath Proof Explorer (MPE) notation.
For information about ND and Metamath, see the
page on Deduction Form and Natural Deduction
in Metamath Proof Explorer.
The original proof, which uses Fitch style, was written as follows
(the leading "..." shows an embedded ND hypothesis, beginning with
the initial assumption of the ND hypothesis):
#  MPE#  ND Expression 
MPE Translation  ND Rationale 
MPE Rationale 
1  1 


Given 
$e 
2  2  

E_{L} 1 
simpld 447 1 
3  11 


E_{R} 1 
simprd 451 1 
4  4 
... 

ND hypothesis assumption 
simpr 449 
5  5 
... 

I 2,4 
jca 520 3,4 
6  6 
... 

I_{R} 5 
orcd 383 5 
7  8 
... 

ND hypothesis assumption 
simpr 449 
8  9 
... 

I 2,7 
jca 520 7,8 
9  10 
... 

I_{L} 8 
olcd 384 9 
10  12 


E 3,6,9 
mpjaodan 764 6,10,11 
The original used Latin letters; we have replaced them with
Greek letters to follow Metamath naming conventions and so that
it is easier to follow the Metamath translation.
The Metamath lineforline translation of this
natural deduction approach precedes every line with an antecedent
including and uses the Metamath equivalents
of the natural deduction rules.
To add an assumption, the antecedent is modified to include it
(typically by using adantr 453; simpr 449 is useful when you want to
depend directly on the new assumption).
Below is the final metamath proof (which reorders some steps).
A much more efficient proof is exnatded9.202 20663.
(Proof modification is discouraged.)
(Contributed by David A. Wheeler,
19Feb2017.) 