Can't use the diacritic character d̤?

Question

I am running Emacs mac ports (24.5) on a Macbook Air. I have the following code as an .el file to setup my own multilingual environment:

(require 'quail)

(quail-define-package "maatra"
                      "UTF-8" "m" t
                      "Input method for Roman transliteration of Devanāgarī using IAST scheme. E.g. aa -> ā"
                      nil t nil nil nil nil nil nil nil nil t)

(quail-define-rules ("aa" ?\ā)
                    ("AA" ?\Ā)
                    ("ii" ?\ī)
                    ("II" ?\Ī)
                    ("uu" ?\ū)
                    ("UU" ?\Ū)
                    (".r" ?\ṛ)
                    (".R" ?\Ṛ)
                    (".rr" ?\ṝ)
                    (".RR" ?\Ṝ)
                    (".l" ?\ḷ)
                    (".L" ?\Ḷ)
                    (".ll" ?\ḹ)
                    (".LL" ?\Ḹ)
                    (".M" ?\ṁ)
                    (".m" ?\ṃ)
                    (".h" ?\ḥ)
                    (".H" ?\Ḥ)
                    (";n" ?\ṅ)
                    (";N" ?\Ṅ)
                    ("~n" ?\ñ)
                    ("~N" ?\Ñ)
                    (".t" ?\ṭ)
                    (".T" ?\Ṭ)
                    (".d" ?\ḍ)
                    (".D" ?\Ḍ)
                    (".n" ?\ṇ)
                    (".N" ?\Ṇ)
                    (";s" ?\ś)
                    (";S" ?\Ś)
                    (".s" ?\ṣ)
                    (".S" ?\Ṣ)
                    ;;("dd" ?\d̤)
                    )

        (provide 'maatra)

I am facing two problems:

1. I receive the error 'Invalid read syntax: ?' only when I uncomment the last line above. Else the code works perfectly. Emacs has no problem with any other character except d̤.
2. Emacs is even unable to properly display the d̤ character except when I use the Monaco font. Other fonts like Dejavu Mono which perfectly display this character in other applications do not render it properly in Emacs. Typically, the adjacent characters overlap this character and the text becomes unreadable.

How do I fix the problem with the d̤ character?

Answer 1

The d̤ is the only character in there which isn't pre-composed; it's a normal LATIN SMALL LETTER D followed by a COMBINING DIAERESIS BELOW. That makes it a syntax error because a character literal can only be a single character. quail-define-rules will also accept strings so use "d̤" instead.