A fragment of a ~/.XCompose file that adds compose-key combinations for typing Greek letters, blackboard bold capitals, and a nonbreaking space.

partial .XCompose

# PKGW customizations
# Greek ambiguities: epsilon/eta, theta/tau, pi/phi/psi, omega/omicron

<Multi_key> <backslash> <comma> : " " U202F # thin nonbreaking space

<Multi_key> <g> <a> : "α"
<Multi_key> <g> <b> : "β"
<Multi_key> <g> <g> : "γ"
<Multi_key> <g> <d> : "δ"
<Multi_key> <g> <3> : "ε" # note! 
<Multi_key> <g> <z> : "ζ"
<Multi_key> <g> <e> : "η"
<Multi_key> <g> <h> : "θ" # note!
<Multi_key> <g> <i> : "ι"
<Multi_key> <g> <k> : "κ"
<Multi_key> <g> <l> : "λ"
<Multi_key> <g> <m> : "μ"
<Multi_key> <g> <n> : "ν"
<Multi_key> <g> <x> : "ξ"
# note: no omicron
<Multi_key> <g> <p> : "π"
<Multi_key> <g> <r> : "ρ"
<Multi_key> <g> <s> : "σ"
<Multi_key> <g> <t> : "τ"
<Multi_key> <g> <u> : "ψ"
<Multi_key> <g> <f> : "φ" # note!
<Multi_key> <g> <c> : "χ"
<Multi_key> <g> <o> : "ω"

<Multi_key> <g> <G> : "Γ"
<Multi_key> <g> <D> : "Δ"
<Multi_key> <g> <H> : "Θ"
<Multi_key> <g> <L> : "Λ"
<Multi_key> <g> <X> : "Ξ"
<Multi_key> <g> <P> : "Π"
<Multi_key> <g> <S> : "Σ"
<Multi_key> <g> <F> : "Φ"
<Multi_key> <g> <U> : "Ψ"
<Multi_key> <g> <O> : "Ω"

<Multi_key> <B> <A> : "𝔸"
<Multi_key> <B> <B> : "𝔹"
<Multi_key> <B> <C> : "ℂ"
<Multi_key> <B> <D> : "𝔻"
<Multi_key> <B> <E> : "𝔼"
<Multi_key> <B> <F> : "𝔽"
<Multi_key> <B> <G> : "𝔾"
<Multi_key> <B> <H> : "ℍ"
<Multi_key> <B> <I> : "𝕀"
<Multi_key> <B> <J> : "𝕁"
<Multi_key> <B> <K> : "𝕂"
<Multi_key> <B> <L> : "𝕃"
<Multi_key> <B> <M> : "𝕄"
<Multi_key> <B> <N> : "ℕ"
<Multi_key> <B> <O> : "𝕆"
<Multi_key> <B> <P> : "ℙ"
<Multi_key> <B> <Q> : "ℚ"
<Multi_key> <B> <R> : "ℝ"
<Multi_key> <B> <S> : "𝕊"
<Multi_key> <B> <T> : "𝕋"
<Multi_key> <B> <U> : "𝕌"
<Multi_key> <B> <V> : "𝕍"
<Multi_key> <B> <W> : "𝕎"
<Multi_key> <B> <X> : "𝕏"
<Multi_key> <B> <Y> : "𝕐"
<Multi_key> <B> <Z> : "ℤ"