troff, macros, copy mode

When troff reads a macro or string definition, it is in ``copy mode''. The following sequences are hot (and must be suppressed by adding a backslash if they're supposed to be copied unscathed, which is usually what you want):

inline number register
inline string registers
inline macro arguments
escaped newlines are stripped
comments are stripped
becomes tab
becomes SOH
becomes \ (backslash)
becomes . (dot)