In preparation
```source("http://macosa.dima.unige.it/r.R")    # If I have not already loaded the library
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
# Other aspects than those discussed in S15

Showing the structure of terms

tree(A*B+C)                  #   (A by B) plus C
# `+`
# `*`
# A
# B
# C
tree(A+B*C)                  #   A plus (B by C)
# `+`
# A
# `*`
# B
# C
tree(A+B*C+D)                #   (A plus (B by C)) plus D
# `+`
# `+`
# A
# `*`
# B
# C
# D
tree( A*(B+C) )              #   A by (B plus C)
# `*`
# A
# `(`
# `+`
# B
# C
# The parenthesis "(" only appears to specify that the 2nd term of * is not a single
# one [this is why it does not appear ")"]

tree( -sqrt(A+B)*C )         #   (opposite (√ (A plus B)) by C
# `*`
# `-`
# sqrt
# `+`
# A
# B
# C
tree( -A-B*C^-D )            # opposite(A) minus (B by (C to the (opposite(D)))
# `-`
# `-`
# A
# `*`
# B
# `^`
# C
# `-`
# D

Trees and other graph with alternative methods to S15, here:

# Let's check the structure of the last graph:
tree( (1^2)*-3/(4-5)+6*7 )
# `+`
# `/`
# `*`
# `(`
# `^`
# [1] 1
# [1] 2
# `-`
# [1] 3
# `(`
# `-`
# [1] 4
# [1] 5
# `*`
# [1] 6
# [1] 7

# An exercise:
# from the releases related to an unknown term go to it and its graph

`+`
`/`
`*`
A
`-`
`(`
`+`
A
B
`(`
`-`
A
`(`
`+`
C
D
E
+
_______________/ \_______________
/                                 E
_______/ \_______
*                 -
__/ \__           __/ \__
A       -         A       +
|                / \
+               C   D
/ \
A   B
+
_______________/ \_______________
/                                 E
_______/ \_______
*                 -
__/ \__           __/ \__
A       -         A       +
|                / \
+               C   D
/ \
A   B
+
_______________/ \_______________
/                                 E
_______/ \_______
*                 -
__/ \__           __/ \__
A       -         A       +
|                / \
+               C   D
/ \
A   B
+
_______________/ \_______________
/                                 E
_______/ \_______
*                 -
__/ \__           __/ \__
A       -         A       +
|                / \
+               C   D
/ \
A   B
+
_______________/ \_______________
/                                 E
_______/ \_______
*                 -
__/ \__           __/ \__
A       -         A       +
|                / \
+               C   D
/ \
A   B
# So:   A*-(A+B)/(A-(C+D)) + E
# Check:

tree( A*-(A+B)/(A-(C+D)) + E )
...

Other examples of use

```