Substitutions

Substitutions are expressions of the form [... => ...] replacing an entity by another.

Variables

Variables can be replaced by any other ray:

print (+f(X))[X=>Y].
print (+f(X))[X=>+a(X)].

Function symbols can be replaced by other function symbols:

print (+f(X))[+f=>f].

We can also omit the left or right part of => to add or remove a head symbol:

print (+f(X); f(X))[=>+a].
print (+f(X); f(X))[=>a].
print (+f(X); f(X))[+f=>].

Galaxy expressions can contain special variables such as #1, #2 or #variable.

These are holes named tokens that can be replaced by another galaxy:

print (#1 #2)[1=>+f(X) X][2=>-f(a)].

This allows, notably, to write parametrized galaxies.