Allow colons in VCF Contig IDs: breakend notation is unambiguous

Breakend notation always includes a ":pos" part, so breakends are
unambiguous even if the "chr" in "chr:pos" also itself contains colons.

As this is a relaxation of the previous rules, there is no concern
about altering all three 4.1/4.2/4.3 specs.

Fixes the VCF/colon aspects of samtools#124. Fixes samtools#258. Closes samtools#291.

VCFv4.1.tex

@@ -157,7 +157,7 @@ \subsubsection{Fixed fields}
 There are 8 fixed fields per record. All data lines are tab-delimited. In all cases, missing values are specified with a dot (`.'). Fixed fields are:
 
 \begin{enumerate}
-  \item CHROM - chromosome: An identifier from the reference genome or an angle-bracketed ID String (``$<$ID$>$'') pointing to a contig in the assembly file (cf. the \#\#assembly line in the header). All entries for a specific CHROM should form a contiguous block within the VCF file. The colon symbol (:) must be absent from all chromosome names to avoid parsing errors when dealing with breakends. (String, no white-space permitted, Required).
+  \item CHROM - chromosome: An identifier from the reference genome or an angle-bracketed ID String (``$<$ID$>$'') pointing to a contig in the assembly file (cf. the \#\#assembly line in the header). All entries for a specific CHROM should form a contiguous block within the VCF file. (String, no white-space permitted, Required).
   \item POS - position: The reference position, with the 1st base having position 1. Positions are sorted numerically, in increasing order, within each reference sequence CHROM.   It is permitted to have multiple records with the same POS. Telomeres are indicated by using positions 0 or N+1, where N is the length of the corresponding chromosome or contig.   (Integer, Required)
   \item ID - identifier: Semi-colon separated list of unique identifiers where available. If this is a dbSNP variant it is encouraged to use the rs number(s). No identifier should be present in more than one data record. If there is no identifier available, then the missing value should be used. (String, no white-space or semi-colons permitted)
   \item REF - reference base(s): Each base must be one of A,C,G,T,N (case insensitive). Multiple bases are permitted. The value in the POS field refers to the position of the first base in the String. For simple insertions and deletions in which either the REF or one of the ALT alleles would otherwise be null/empty, the REF and ALT Strings must include the base before the event (which must be reflected in the POS field), unless the event occurs at position 1 on the contig in which case it must include the base after the event; this padding base is not required (although it is permitted) for e.g. complex substitutions or other events where all alleles have at least one base represented in their Strings.  If any of the ALT alleles is a symbolic allele (an angle-bracketed ID String ``$<$ID$>$'') then the padding base is required and POS denotes the coordinate of the base preceding the polymorphism. Tools processing VCF files are not required to preserve case in the allele Strings. (String, Required).

VCFv4.2.tex

@@ -174,7 +174,7 @@ \subsubsection{Fixed fields}
 There are 8 fixed fields per record. All data lines are tab-delimited. In all cases, missing values are specified with a dot (`.'). Fixed fields are:
 
 \begin{enumerate}
-  \item CHROM - chromosome: An identifier from the reference genome or an angle-bracketed ID String (``$<$ID$>$'') pointing to a contig in the assembly file (cf. the \#\#assembly line in the header). All entries for a specific CHROM should form a contiguous block within the VCF file. The colon symbol (:) must be absent from all chromosome names to avoid parsing errors when dealing with breakends. (String, no white-space permitted, Required).
+  \item CHROM - chromosome: An identifier from the reference genome or an angle-bracketed ID String (``$<$ID$>$'') pointing to a contig in the assembly file (cf. the \#\#assembly line in the header). All entries for a specific CHROM should form a contiguous block within the VCF file. (String, no white-space permitted, Required).
   \item POS - position: The reference position, with the 1st base having position 1. Positions are sorted numerically, in increasing order, within each reference sequence CHROM.   It is permitted to have multiple records with the same POS. Telomeres are indicated by using positions 0 or N+1, where N is the length of the corresponding chromosome or contig.   (Integer, Required)
   \item ID - identifier: Semi-colon separated list of unique identifiers where available. If this is a dbSNP variant it is encouraged to use the rs number(s). No identifier should be present in more than one data record. If there is no identifier available, then the missing value should be used. (String, no white-space or semi-colons permitted)
   \item REF - reference base(s): Each base must be one of A,C,G,T,N (case insensitive). Multiple bases are permitted. The value in the POS field refers to the position of the first base in the String. For simple insertions and deletions in which either the REF or one of the ALT alleles would otherwise be null/empty, the REF and ALT Strings must include the base before the event (which must be reflected in the POS field), unless the event occurs at position 1 on the contig in which case it must include the base after the event; this padding base is not required (although it is permitted) for e.g. complex substitutions or other events where all alleles have at least one base represented in their Strings.  If any of the ALT alleles is a symbolic allele (an angle-bracketed ID String ``$<$ID$>$'') then the padding base is required and POS denotes the coordinate of the base preceding the polymorphism. Tools processing VCF files are not required to preserve case in the allele Strings. (String, Required).

VCFv4.3.tex

@@ -224,7 +224,7 @@ \subsubsection{Contig field format}
 \end{verbatim}
 
 \noindent
-Valid contig names must follow the reference sequence names allowed by the SAM format ("{\tt [!-)+-\char60\char62-\char126][!-\char126]*}") excluding the characters "\texttt{\textless\textgreater[]:*}" to avoid clashes with symbolic alleles and breakend notation.
+Valid contig names must follow the reference sequence names allowed by the SAM format ("{\tt [!-)+-\char60\char62-\char126][!-\char126]*}") excluding the characters "\texttt{\textless\textgreater[]*}" to avoid clashes with symbolic alleles.
 The contig names must not use a reserved symbolic allele name.
 
 
@@ -286,7 +286,6 @@ \subsubsection{Fixed fields}
 \begin{enumerate}
   \item CHROM --- chromosome: An identifier from the reference genome or an angle-bracketed ID String (``$<$ID$>$'') pointing to a contig in the assembly file (cf. the \#\#assembly line in the header).
   All entries for a specific CHROM must form a contiguous block within the VCF file.
-  The colon symbol (:) must be absent from all chromosome names to avoid parsing errors when dealing with breakends.
   (String, no white-space permitted, Required).
   \item POS --- position: The reference position, with the 1st base having position 1.
   Positions are sorted numerically, in increasing order, within each reference sequence CHROM.
@@ -2033,6 +2032,10 @@ \subsection{Changes to VCFv4.3}
 \begin{itemize}
 \item More strict language: ``should'' replaced with ``must'' where appropriate
 \item Tables with Type and Number definitions for INFO and FORMAT reserved keys
+
+\item
+The VCF specification previously disallowed colons (`{\tt :}') in contig names to avoid confusion when parsing breakends, but this was unnecessary.
+Even with contig names containing colons, the breakend mate position notation can be unambiguously parsed because the ``{\tt :}\emph{pos}'' part is \textbf{always} present.
 \end{itemize}
 
 \subsection{Changes between VCFv4.2 and VCFv4.3}