Unicode characters can either be encoded directly in the document or accessed by reference.
You need a unicode-aware editor to encode them directly and then you just type in the character you want.
Otherwise you use a character entity reference:
[66] CharRef ::= '&#' [0-9]+ ';' | '&#x' [0-9a-fA-F]+ ';'
An example, some mathematics:
∀α∈Γ(...) ∀α∈Γ(...) |
∀α∈Γ(...) |